I think an important part of the "D&D is easy to learn" argument is that a lot of those people don't actually know how to play D&D. They know they need to roll a d20 and add some numbers and sometimes they need to roll another type of die for damage. A part of it is the culture of basically fucking around and letting the GM sort it out. Players don't actually feel the need to learn the rules.
Now I don't think the above actually counts as knowing the rules. D&D is a relatively crunchy game that actually rewards system mastery and actually learning how to play D&D well, as in to make mechanically informed tactical decisions and utilizing the mechanics to your advantage, is actually a skill that needs to be learned and cultivated. None of that is to say that you need to be a perfectly tuned CharOp machine to know how to play D&D. But to actually start to make the sorts of decisions D&D as a game rewards you kind of need to know the rules.
And like, a lot of people don't seem to know the rules. They know how to play D&D in the most abstract sense of knowing that they need to say things and sometimes the person scowling at them from behind the screen will ask them to roll a die. But that's hardly engaging with the mechanics of the game, like the actual game part.
And to paraphrase @prokopetz this also contributes to the impression that other games are hard to learn: because a lot of other games don't have the same culture of play of D&D so like instead of letting new players coast by with a shallow understanding of the rules and letting the GM do all the work, they ask players to start making mechanically informed decisions right away. Sure, it can suck for onboarding, but learning from your mistakes can often be a great way to learn.
