Not familiar with the novel but a quick look at the Wikipedia page indicates that the work was inspired by H. Rider Haggard, a 19th century English novelist and this might be in relation to the controversy over Great Zimbabwe. One of Haggard's famous works was King Solomon's Mines (1886) which popularized the idea that the recently discovered (by Europeans) site of Great Zimbabwe was of non-African origin and was the basis of the biblical story of Solomon and the Queen of Sheba; it seems the author of The Sunbird, Wilbur Smith was also inspired by this story. Haggard was merely replicating the views of the German traveler, Karl Mauch who originally located the place in 1871 and speculated that this was Ophir, the location of King Solomon's mines. Mauch and Haggard's view of a non-African origin to Great Zimbabwe was confirmed by the the archaeologist James Theodore Bent who excavated the site in the 1890s, though his method wasn't the most scientifically rigorous. Despite uncovering artifacts consistent with the material culture of the local peoples, Bent dismissed the idea that the site had a domestic origin and attempted to link it to any number of Middle Eastern and Mediterranean civilizations (including the Egyptians, Assyrians and Arabs). Any number of archaeologists and looters followed Bent in desecrating the site for valuables (this pillaging was sanctioned and licensed by Cecil Rhodes and his Rhodesia Ancient Ruins Ltd.), removing many artifacts as well as knowledge; any conclusions made about the site during this time naturally supported the non-African origin and several other civilizations were thrown into the mix as the founders of Great Zimbabwe such as the Phoenicians. Despite the damage, proper archaeological work was, nonetheless, conducted under Randall MacIver in 1905 and later Gertrude Caton-Thompson in 1929 and they both supported the view that Great Zimbabwe was a product of the local peoples.

In short, The Sunbird is merely repeating discredited theories posed by untrained European travelers and 'archaeologists'. This view that Great Zimbabwe was of non-African origins also had a political dimension as it justified the British colonial (and later, Rhodesian) rule of the region under the guise of bringing civilization to a location where it was previously absent. A domestic civilization (as evidenced by Great Zimbabwe and many other local sites such as Khami and Dhlo-Dhlo/Danagombe) capable of sophisticated architecture and possessing an equally sophisticated material culture naturally puts a damper on such delusions.

Sources:

Shillington, Kevin. Great Zimbabwe Colonial Historiography. Encyclopedia of African History: A - G.. 1, Volume 1.

Hall, Martin. Great Zimbabwe and the Lost City: The cultural colonization of the South African Past. Found in Theory in Archaeology: A World Perspective

Robinson, Michael F. *The Lost White Tribe: Explorers, Scientists, and the Theory that Changed a Continent *

I have read the book and enjoyed it for what it is, but it is not based on a proper theory or any real evidence whatsoever. Wilbur Smith, unfortunately, was working from a popular belief of his childhood that Sub-Saharan Africans could not be responsible for the ruins of Zimbabwe because they had no tradition of masonry that survived to the present. The few people who took an interest in the antiquity of that region during the Scramble for Africa preferred to believe that Phoenicians had made their way inland and built a civilization there as a nation of white people surrounded by Black Africa. They based this idea on the fact that Phoenicians had circumnavigated the continent once on the order of Pharaoh Necho II, and Carthaginians --basically just the Western Mediterranean Phoenicians-- had sailed down the west coast of Africa at least as far as Sierra Leone.

To their minds, Occam's Razor said it was more likely that the Phoenicians had made their way deep into the heart of the continent, forgotten their own traditions of masonry and pottery, and left no trace of their culture or language or descendants on the population of today, rather than imagine Africans might have invented masonry for themselves during a population boom supported by the exploitation of local gold reefs and then lost the art during the decline in population when the reefs were exhausted.

You can see the idea sits uneasily on Smith's mind even in the novel, as in the last act he has the conquering army of Africans gather up all trace of their vanquished Phoenician overlords and throwing them piece by piece down a bottomless water-filled sinkhole until only Zimbabwe remains. That's how Smith worked around the lack of any archaeological remains supporting the idea of a Phoenician origin to the lost civilization.

I have read some books that make a half-hearted argument that the Phoenicians might have had a hand in Zimbabwe: They were written in the 40s and 50s, and they advance the notion as one of several possible explanations that might be true. I am always conscious of how prevalent racism was at this time: I have read another book from roughly the same era that mentions the hypothesis that Negroes may have evolved from Homo Habilis while the rest of humanity evolved from a different member of the Homo genus. What was the basis for that argument? Why the sloping brow and large jaw, of course! (Please read that with an appropriate tone of exasperated sarcasm.)

It was very popular in the time of Smith's childhood to minimize or explain away African accomplishments. These ideas were largely born of racism, but there was also a heavy political element involved as well: By the time the book was being written, Rhodesia was in the midst of a civil war between its white minority population and the black majority who would eventually rename the country Zimbabwe after their victory. Do you imagine many well-educated people were arguing that the ruins of Zimbabwe were built by Africans and that Africans had claim to high civilization in the region dating back hundreds of years before the arrival of Europeans? Much more palatable to argue that foreign civilized people had a history of arriving in the region and trying to bring the benefits of their advanced technology to the sea of black humanity around them.

For the record, I believe Smith was a progressive in his day. Many of his books were banned in Apartheid South Africa for their sympathetic depiction of blacks. That said, he was a product of his generation, and on this topic more than most he just got it plain wrong.

EDIT: Minor edits for clarity.