FWIW, here's what Mary Lefkowitz has to say on the subject:

For centuries Europeans had believed that the ancient historians knew that certain Greek religious customs and philosophical interests derived from Egypt. But two major discoveries changed that view. The first concerned a group of ancient philosophical treatises attributed to Hermes Trismegistus; these had throughout the Middle Ages and into the Renaissance been thought of as Egyptian and early. But in 1614 the French scholar Isaac Casaubon demonstrated that the treatises were actually late and basically Greek. The second discovery was the decipherment of hieroglyphics, the official system of Egyptian writing, completed by 1836. Before decipherment, scholars had been compelled to rely on Greek sources for their understanding of Egyptian history and civilization. Once they were able to read real Egyptian texts, and could disregard the fanciful interpretations of hieroglyphics that had been circulating since late antiquity, it became clear to them that the relation of Egyptian to Greek culture was less close than they had imagined. Egyptian belonged to the Afroasiatic language family, while Greek was an Indo-European language, akin to Sanskrit and European languages like Latin.

On the basis of these new discoveries, European scholars realized that they could no longer take at face value what Herodotus, Diodorus, and the Church fathers had to say about Greece’s debt to Egypt. Once it was possible to read Egyptian religious documents, and to see how the Egyptians themselves described their gods and told their myths, scholars could see that the ancient Greeks’ accounts of Egyptian religion were superficial, and even misleading. Apparently Greek writers, despite their great admiration for Egypt, looked at Egyptian civilization through cultural blinkers that kept them from understanding any practices or customs that were significantly different from their own. The result was a portrait of Egypt that was both astigmatic and deeply Hellenized. Greek writers operated under other handicaps as well. They did not have access to records; there was no defined system of chronology. They could not read Egyptian inscriptions or question a variety of witnesses because they did not know the language. Hence they were compelled to exaggerate the importance of such resemblances as they could see or find.