[Sniffnoy]

> The writer is far more knowledgeable than me, and it seems he believes there was no major decline related to the fall of Rome,

He doesn't claim that at all. In the intro he explicitly claims that there was such a decline and fall.

In the body of this first post of the series, he explores the areas where the case against his claim is strongest. That doesn't mean he's suddenly retracted his claims from the intro.