Ath'kethin
Elder Thing
This statement isn't wrong, exactly, but I think it's a little misleading. Basic D&D did lack some of the funhouse aspects of AD&D out of the box, but it made it incredibly easy to add additional classes (remembering that, as you said, race and class were largely indistinguishable in that version of the game).Now, Mystara is an interesting case as it was the setting for classic D&D, and those rules had fewer options in core than AD&D, plus races were their own class.
It was also MORE allowing in some ways. In Basic, for example, almost any character could have almost any class abilities; it was just necessary to add them (creating a new class), and that was definitely NOT true of AD&D as written. The best example of this disparity IMO was King Kol, the kobold wizard-prince in Glantri, and how they handled him when Mystara was converted to AD&D 2e.
In Basic, a kobold wizard was no big deal, but in AD&D kobolds Could Not Be wizards, which caused a problem. So they decided that Kol wasn't ACTUALLY a kobold, he was a small and shriveled elf who had somehow trucked everyone into thinking he was a kobold. It was an approach both puzzling and insulting, but that was that. The class restrictions seemed an odd hill to die on, but someone at TSR decided that Mystara would, indeed, die on that hill. And so it did.