Menachem, look at the meforshim on that medrash rabba – aotz yosef, etc…they all say it isn’t literal, because of all the many places where chazal say that moshe knew everything. That sefer smerel quoted brings a leshem which says that someone who interprets the medrash that way has veered off the path – and that’s a nistar source, too.
No pilpul in the world will turn the rambam against his own words. He says not to look at a mezuzah as a shmirah by itself. Anything else is twisting his words.