Diamond is "accurate" in the sense that the horserider theory definitely exists. However, the horserider theory is not really a theory of Japanese origins; rather, it is a theory of how a "Japanese" state came to be in relation to activities elsewhere in Asia, and particularly activities in the Korean peninsula. (I use "Japanese" in scare quotes because the thing we call "Japan" is wholly a modern formulation and has no meaning in the time period we are discussing).

William Farris does a good job of explaining what the horserider theory is and how it came to be:

In the ruins of postwar Japan, rejection of the prewar ideology and criticism of previous scholarship were universal, and these trends affected how historians saw ties to the ancient Korean states. Iconoclastic interpretations like Tsuda's came into vogue in the late 1940s, but nothing could have prepared scholars for the shocking views first voiced by Egami Namio in 1948. [12] As an ethnologist and archaeologist, Egami created his own unorthodox periodization of the great tombs of Japan by dividing them into early and late eras. He then argued that the archipelago had witnessed a sudden invasion by militaristic nomads of North Asia at the end of the first period in the early fourth century. These aristocratic equestrians had ridden down the peninsula and crossed the Korean Straits in boats, founding kingdoms in Korea and Japan as they went. Egami also used the Japanese classics to support his contentions, even naming the tenth Emperor Sujin ("Mimaki") as the foreign conqueror of Japan.

The horserider theory was a brilliant synthesis of archaeology, history, and ethnology and it had at least three attractions. First, it attempted to explain a period for which there was no written evidence to contradict the thesis. Second, it seemed as though there was some truth to the archaeological trends that Egami proposed, such as the influx into Japan of horse-riding equipment and massive amounts of weapons during Egami's late Tomb period. Third and most important, the theory linked Japan to Korea and the peoples of Northeast Asia. Japanese scholars had previously proposed such a connection, calling the hypothesis Nissen doso ron ("Japanese and Koreans have the same ancestor"). The horserider hypothesis addressed the problem of why the Japanese court of the sixth, seventh, and eighth centuries fought so hard to retain its base in southern Korea by claiming that the leaders of Japan, Paekche, Silla, and Kaya were all relatives.

Farris also explains the politics behind the emergence of Egami's horserider theory:

Thanks to Egami's untiring defense, the horserider theory continues to be influential. Although the overwhelming majority of Japanese scholars have rejected it, the thesis gained popularity among the growing community of scholars of Korean history in Korea, Europe, and the United States.[14] These scholars came to view the horserider theory as a necessary antidote to insular, chauvinistic interpretations put forward in Japan.

Ultimately, however, scholarly consensus has not supported the horserider theory. Farris explicates several reasons to reject the horserider theory in light of the archaeological record, and he describes these reasons as such:

While there is no doubt that horse gear and riding skills entered Japan from southern Korea, the evidence does not fit Egami's horserider theory. Trappings appeared too late to have been associated with an invasion in the mid- or late fourth century; moreover, their spread across Japan was too gradual. [71] There are at least two other important reasons for rejecting the horserider theory: Japanese did not practice castration of livestock as in the Asian steppe, and Egami's characterizations of agricultural peoples as pacific and equestrians as warlike are inaccurate stereotypes. [72] Nomads did not gallop through Korea and Japan founding kingdoms, but inhabitants of Korea did play an essential role in transferring horse-riding technology to Japan...

Besides, many other monuments and artifacts in Japan, from the characteristic keyhole shape of tombs to the size, material, and structure of coffins, show strong continuities over the period when horseriders were supposed to have been radically transforming culture in the islands.

Source: Farris, William Wayne. Sacred Texts and Buried Treasures: Issues in the Historical Archaeology of Ancient Japan. Honolulu: University of Hawaii Press, 1998.

The most recent DNA data is showing that the modern Japanese are descended from a mixture of rice agriculturalists who arrived in Japan circa 400BC and the prior indigenous residents of the Japanese islands, who had been there for thousands of years. So, theory #4. But current inhabitants of Japan have much more genetic material from people of the Asian mainland than from the indigenous groups. Plus, the amount of genetic material from the indigenous groups is greater the further north you get from Kyushu. It looks like the Ainu, who mainly live on the northern Japanese island of Hokkaido, are the remnants of the indigenous people who were in the islands pre-invasion. Basically, rice agriculturalists have a much higher birthrate than nomadic or hunter-gatherer peoples, so they quickly were able to outpopulate the indigenous inhabitants. They also had some technology that the indigenous inhabitants did not have. They didn't entirely destroy the indigenous people, but they did affect their population greatly.

The horse-riding nomads idea is based partly on archaeological evidence--there was a major shift in material culture in the Japanese islands starting around 400-300BC, and it looks like it originates at the part of the Japanese islands that are closest to the mainland of Asia--Kyushu. Measurements of skeletons shows a change in some of the elements, as well. The Korean peninsula is the closest land to the Japanese Islands, so for a variety of reasons, Korea as the origin of the horseriding peoples makes the most sense. However, there was a lot of anti-Korean sentiment at the time this theory originated, so the idea that the Japanese people originated with Koreans was not popular, hence the emphasis that the invading peoples were not Korean (of course they weren't technically Korean anyway--Korea did not exist 2000 years ago. But yeah, they were most likely indigenous inhabitants of the Korean peninsula). The horseriding element has also been overemphasized in the sense that, yeah, horses were important, but there were other elements that were important as well. But it's a convenient shorthand to refer to the invading peoples as horseriders.

The Japanese language is a language isolate--there is no language close enough to it to aid in determining origins of the language or any languages being related to it.

In any event, it's important to understand the politics behind these theories. During the first half of the 20th century, there were political forces who were invested in emphasizing that the Japanese people were unique and special, that the imperial family had important and unique origins, and that the Japanese were meant to rule over Asian, not that they were a peripheral offshoot of some random tribe of common invaders from Asia. Japan had annexed Korea and Koreans were inferior people from a backwards area, to the Japanese political propaganda of the time. By locating the origins of the Japanese people in some kind of mysterious, ancient past, helped to elevate the Japanese people with a unique origin story. Beginning in the 1970s and continuing until today is a style of lightweight non-fiction in Japan called Nihonjinron, in which the unique and special qualities of the Japanese people and the Japanese islands are emphasized, usually with pretty shaky scientific backing. The de-emphasis of Korea as an origin point is popular in this context.

May I ask how well regarded is Diamond in this subreddit? I know that quite a few historians and anthropologists discredit his environmental determinalism for various reasons, and claim that Guns, Germs, and Steel is not a well-researches or accurate work. Personally, I very much enjoyed it because it helped me begin to think about things like that, about how people came to be where they were and not just as they are. I had an anthropology teacher, a geography professor, and an archaeology professor who all regarded GGS and Diamond highyl, though each acknowledged and discussed the flaws I mentioned plus more I can't remember. The archaeology professor even used GGS as text material for his class, along with Robert Wright's Nonzero, which is sort of an opposite yet parallel perspective on the same topic.

I'm just curious as to how Diamond's work in general is viewed here.

Just a small followup question regarding Diamond, because I've got a few books of his as Christmas gift. What is overall opinion of him and his works? From some responses here I can already see that if something particular will catch my interest I will need to do further research, but still I would like to know how much trustworth he is.