I got into the gist of the Mongol invasions of Japan, where I stumbled upon a piece (lacking citation) on Wikipedia saying:
The invasions exposed the Japanese to an alien fighting style which, lacking the single combat that characterized traditional samurai combat, they saw as inferior. The Mongol method of advances and withdrawals based on signal sounds from bells, drums and war cries was also unknown in Japan at that time, as was the technique of shooting arrows en masse into the air rather than long-ranged one-on-one combat.
(emphasis mine)
Is this true - especially the use of "arrow rain" so to speak - and if so, how did the Japanese fight in unity before discovering these basic methods? How is it possible that they could be so far disconnected from the (then) contemporary standard of war and formations?
Is there any ground to the idea that the Japanese were still fundamentally cut off from the rest of the world and so were lacking in warfare?
I wrote before about early samurai combat here and here. In short, samurai did not engage in duels during battle, but they did fight by taking individually-aimed shots. Whether that counts as "one-on-one" combat is up to you. I do want to point out shooting in arc is generally for skirmishing and could be done (there are accounts of arrows exceeding 400m, and the maximum range of Japanese bows approaches 400m). However such shots would not have been very effective, due to loss of force in flight and loss of accuracy. Samurai would've needed to shoot at close to 1314m (according to Thomas Conlan) to penetrate an opposing samurai's armour. So it's not really that weird that the estimate range when samurai first shot at the invasion force (based on Takezaki Tsunenaga's illustrations) was 2030m.
The lack of sound signals from bells, drums, and trumpets seem to be true (war cries is not an effective way of communication, and certainly Tsunenaga depicted him and others saying things mid-combat). From Tsunenaga's scroll the main method of communication seem to have been to rally around a flag.
As for why the Japanese did not use musical instruments to communicate, the most likely answer is it wasn't seen as necessary with flags already being used. Bells and drums are mentioned in Classical Japanese sources, but since the appearance of the samurai, the only large scale warfare before the Mongol invasions was Jishō-Juei War (Genpei War), when accounts of army sizes were most likely inflated. Heck Conlan estimates that the defenders of the Mongol Invasion numbered in the thousands (though admittedly Conlan's the low end of the estimate). Flags are plenty enough for an army of this size, when units would be in the low hundreds or smaller.
Were Japanese "lacking in warfare?" The above passage was likely taken out of the opening paragraphs for the 1274 combat in the Hachiman Gudōkun. But it also says that whenever the invaders retreated they lobbed bombs to cover their retreat and disorient the Japanese. Sure, that's a nice use of gunpowder weapons that must have been novel to the Japanese, but don't overlook the fact that this line also means that during combat at least some units of the invasion force were put to flight. Japanese sources agree they won at least some skirmishes in 1274, but (according to the Hachiman Gudōkun) decided to pull back to Dazaifu due to casualties. Both Japanese and Korean sources state that a major Chinese commander was wounded in combat. Korean sources state that after the landing combat, the men were exhausted, and Chinese source say despite victory in combat they had spent their arrows and could not get organized, so both sources imply that, despite defeating the Japanese in battle, heavier-than-expected resistance forced them to withdraw in 1274. And the Korean contingent of the 1281 invasion couldn't even make successful landing. So was Japan lacking in warfare? Not really.