There are primary sources. There are archeological sites:
https://en.wikipedia.org/wiki/Qianling_MausoleumThere are also histories written just after the Tang Dynasty had fallen. It was considered good form for a new dynasty to write a historical review of the previous one, and there are two retrospectives on Tang, e.g.: https://en.wikipedia.org/wiki/Old_Book_of_Tang
And there is ironclad evidence from foreign sources. Among others, Japanese histories detail the journeys of emissaries to the Tang Court: https://web.archive.org/web/20260315181529/https://japansoci...
So the theory is bait, and hard to take seriously.
But I'll give it credit for hijacking a genuinely interesting scholarly premise: What is taken quite seriously is the notion that China's first dynasty -- the Xia Dynasty -- never existed. It is of rather mythic proportions; later Chinese would speak of Xia the way Romans would speak of Troy. And there is very little evidence to support its existence. The Shang Dynasty is also supported by rather scant evidence -- and is also mythic, though obviously less so than the Xia.
Worth noting that both the Xia and Shang are exceedingly ancient -- roughly 2000 BC to 1000 BC between them -- unlike the Tang, which fell around 900 AD and is thus middle-of-the-pack by Chinese dynastic standards.