He used synchronisms and 'hard' dates. Ramses "the Great" was rc dated by his guts (from inside one of his canopic jars) and was, as predicted, (V said "nearly 800 years") younger than the conventional pseudochronology must have him be. Soooo, the results were published and the spurious excuse was, the jar (the second jar had an aromatic salve) was reused. BTW, despite any claim to the contrary, there had never previously been any doubt that they were Ramses II's, and afaik there's zero evidence that any pharaonic-era canopic jars were ever reused in a later burial. And in the Theses, #245 -- "Tiles of buildings erected by Ramses II (in Kantir) which have Greek letters on the back, are products of Greek laborers in the service of the pharaoh. The letters are genuine Greek letters of the sixth century."
Yep, that was the sort of vagary I recalled. It’s a pity that contemporary historians don’t take this evidence seriously (inventing rationalizations to keep things as they are.) Otherwise they’d at least be able to provide a single range of dates for the actual dynasties rather than live with what looks like twin time existences for some of them separated by centuries.