Yitay,
I used that example because it is one of the examples of a pirush that is halacha l’Moshe miSinai used in the Rambam in his hakdoma to Pirush HaMishnayos.
I personally think that although it is not explicit, it is implied from the Gemara (which is trying to explain a makor for the stam din in a Mishna) that this was a tradition and that if there is a the machlokes is where the smach to this tradition is in Torah (which is why everyone agrees that it means money despite not agreeing on a makor in the posuk).
It is possible that someone might disagree with the Rambam as to the status of this din, and rather holds it was derived in an honest attempt to derive the meaning of the Torah. I don’t think that would be apikorsus. But saying that the Torah was meant literally and the Sages changed the law because they didn’t like it and then lied and said it was what the Torah really meant, is apikorsus.