Isaac Casaubon's 1614 case against the Hermetica's ancient Egyptian authorship was philological rather than doctrinal, which is why it held. He identified Greek philosophical vocabulary and argumentative moves in the texts that belong demonstrably to the Hellenistic and early Roman periods, along with linguistic features inconsistent with the archaic Egyptian date the texts had been assigned, and laid the evidence out systematically enough that even scholars invested in the older dating could not answer it on its own terms. The texts were not, as claimed, the work of an Egyptian sage predating Moses and standing behind Plato; they were composed centuries after Plato, by writers working within a Greco Egyptian milieu of the Roman imperial period.
The finding arrived too late to affect how earlier Renaissance readers had already used the texts, since those readings were built a full century or more before Casaubon wrote, treating the supposed antiquity of Hermes Trismegistus as license to read Hermetic material as a philosophical and even proto Christian revelation. Casaubon's dating did not undo that reception history, it simply meant every subsequent reader encountered the Hermetica already knowing the ancient authorship the earlier reception had found so authoritative was itself a mistake, without the texts' content as Greco Egyptian religious philosophy becoming any less real for that correction.