> Denying the existence of a major dynasty is like declaring that Abraham Lincoln and Andrew Johnson didn't exist

There's a great difference of scale.

Lincoln died about 150 years ago. There are voluminous primary sources about Lincoln - e.g. photographs of him, original documents that he wrote, firsthand accounts of his actions, etc. We certainly don't know every detail of his life, but we do know a lot of it, and we're quite confident it's true.

The Tang dynasty, by contrast, fell over a thousand years ago. We know of it primarily through second- and third-hand histories and through interpretation of archaeological artifacts - much like our knowledge of Greek and Roman history, for instance. There are a lot of thinly covered areas, and many of the sources are biased; there's a lot more to be uncertain about.