Found this Ibn Ezra quote on the Nishmat
website. He is talking about the midrash of the assimilated Jews being killed during the Plague Of Darkness, and he mentions the fact that this midrash plays into the hands of "Moslem scholars" who were already attacking the rationality of the Exodus story.
A derash says one in five hundred left. This is a lone opinion, which is debated, and is not at all a received tradition [which would be binding]. We already have enough grief from the Moslem scholars, who say: "How is it possible that fifty-five men who went down to Egypt [with Yaakov] could be the ancestors, over 210 years, of 600,000 men over the age of twenty?"<8> ... In [every] plague that consumed Egypt, the Jews were spared. Most of the Egyptians' cattle died during dever (the plague of cattle-disease), while not one of the Jews' cattle died (Shemot 9:6). In the Plague of the Firstborn, not a single Jew died. In the Plague of Darkness, it says, "There was light for all the Jews" (Shemot 10:23). In the plagues in which Egyptians died, Jews did not die. So in a plague (darkness) in which no Egyptian died, how could all the Jews die until only one in five hundred survived? [Besides, that would mean] the Jews did not have light in their homes, but the darkness of disease and the pitch-black of death! Since only a tiny part was left from a huge nation, this would not have been redemption for the Jews but a sick evil! That's the opposite of the text. The whole thing is a derash; don't rely on it. Maybe the one who said it at the outset had a hidden reason (sod).