Rabbi Menachem Meiri [
Beit HaBechirah, Pesachim 109:

In several places we have explained that in those times people were drawn after popular notions, such as incantations, omens, and folk practices. Anything that did not involve idolatry or the ways of the Amorites, the Sages were not concerned to uproot. All the more so regarding things to which they were so accustomed that their nature accepted them in the sense that the masses believed that these practices could strengthen or weaken them through incantations, etc., to the point that when they said or performed them they felt strengthened, and in their absence they felt weakened. As is stated in this passage (Pesachim 110b): "If one is particular, they are particular with him; if one is not particular, they are not particular with him"—one who is particular, meaning one who is concerned to observe matters of incantations, omens, and the like, and this time was not particular, the demons are particular with him and harm him. But one who does not observe the "rules" of incantations, omens, and the like, the demons are not particular with him. Among these things was their practice of being careful about "pairs," for example, not to drink an even number of cups. And when the Sages instituted four cups, they said not to reduce or add, but to drink exactly four cups, even though according to that nonsense it is an even number. Because of how accustomed people were to this, they needed to give a reason for their words, since those absurdities were ingrained in the people. The Sages were therefore forced to explain why there is no concern on the night of Passover about "pairs," and that is what they said (Pesachim 109b): "It is a night of watching"—a night protected from harmful forces …

Can you point me to sources that understand "a law given to Moses at Sinai" as you mentioned? Thanks.

For example, the Rosh's commentary on Niddah, Laws of Mikvaot 1 (regarding three log of drawn water), Ritva at the beginning of chapter 2 of Eruvin (regarding the boards around wells). I also recall a Tosafot that cites a Mishnah in Parah about a law given to Moses at Sinai and explains that it is a rabbinic law (though right now I found only Bekhorot 54, which speaks about liability for death and not about a law given to Moses at Sinai).

Did the medieval authorities who said that "a law given to Moses at Sinai" is not literal say that only about rabbinic laws? Meiri is saying nonsense about the testimony of an Amora that the Talmud does not dispute. More than that, it even expands on his words because it is not clear what he meant by the third example he brought.

Why does that matter? If they said it about rabbinic laws, why shouldn't they say it about a popular folk belief? Maimonides writes similar things even about actual laws (like the evil eye and washing the hands, and more).

It seems to me that there is an essential difference. An Amora testifies that this is a law given to Moses at Sinai, and the Talmud accepts it. Meiri does not say that there is anyone who disagrees; he just calls it nonsense. I'm looking for some explanation of his position.