^Which does not in any way lend credence to the guess that "Class M" had anything to do with Mars. I mean, that's a completely illogical conclusion. If he had intended it to be the initial of a habitable world, obviously he would've gone with "Class E" for Earth.
I don't agree.
Both the pitch and the writers guide equated "Class M" with "Earth-Mars conditions". There's a big fat "M" right there in "Earth-Mars conditions", so it's a logical suspect for where the "M" came from. Barring any further evidence, it's nothing more than that: a logical suspect. However, Darkwing
may even have guessed it correctly as a contraction of "EM-Class".
Now, as to why "Earth-Mars conditions" wound up in the pitch, maybe Roddenberry put it in because he was initially going to make Spock half-Martian.