^"Not everything has to tie in?" An odd comment, given that the whole point of ENT was to show the origins of the Trek universe we knew. Naturally the intention was that "Minshara-class" would be the designation that eventually got shortened to M-class. I liked it. I liked the idea that something in standard Federation usage originated with the Vulcans instead of everything being Earth-based. ENT did a very good job of breaking free of that "everything is human in origin" assumption and showing that some familiar things originated with other species -- for instance, Vulcans and Andorians having deflector shields and tractor beams before Earth did, Klingons and Vissians having photon(ic) torpedoes first, Xyrillians inventing the holodeck, etc.
And no, we do not know canonically
that these classes are just parts of an alphabet scale. That's a fan assumption, but we don't have enough letters canonically specified to know that for sure. Assumptions are not inviolable truths. And of course, even if there is an alphabet scale in the 23rd or 24th century, they could easily have chosen the letter M to represent Earth/Vulcan-like planets because
of the established "Minshara-class" usage. That would make perfect sense and it's a bizarre thing to get angry about.