D&D games have DMs, the D&D universe doesn't.
Literally no one has claimed that or anything tantamount to that. Objections were raised on two topics: are D&D artifacts and god created items equivalent, and whether or not the Marvel Astral Plane is the same as the D&D one.
I would be willing to accept the bit about artifacts and I could accept that Dr. Strange's spells are fundamentally different from D&D ones. But the Astral Planes being the same and AMF not blocking Stange's (non artifact) magic is where I disagree. Neither of those claims make any sense.