I mean, they're not really wrong. D&D is not and never has been the Everything Game. It doesn't do all genres equally well and not all of those optional rules variants in the DMG work particularly well. Anyone who tries to sell the idea that it is the Everything Game is someone who just really doesn't want to switch systems, even if they have to bury it under a binder of house rules thicker than the PHB.
D&D is going for the mass market appeal, and that leaves plenty of space for smaller competitors to claim the less trafficed niches for themselves. Many of which are offering games very similar to D&D, be it 5e or earlier editions, if you don't want to stray too far. But D&D is the biggest game in the field precisely because it does go for the mass market, and right now that's pretty clearly a high magic flavor.