I would say he's grasping for an explanation and, not surprisingly being a game designer, looking to the rules.
That's not it though in my view. The rules can either support or get in the way of telling the sort of tale he experienced, but that's about it.
That sounds like a game-designer, too - just one more of the storytelling school.
It's the archetypal story, not the rules, that creates this emotional response. Dungeons are representations of the Underworld, the place Heroes - amalgams of the best of human traits - go to face Dragons, symbols of chaos that represent an amalgam of the predators that preyed on early humans. This is one of the oldest stories known to exist and we keep telling ourselves this story because it's deeply important psychologically.
That's not the D&D experience, though. Back in the day, you mapped the dungeon, evaded or ambushed monsters, worried constantly about traps, possibly betrayed your buddies, and made away with treasure. Your character didn't amalgamate the best of human traits, you were lucky if you had one or two good stats, and probably had one horrible one, at least. You had a specialty that you could do that was niche-protected so others couldn't.
Now, yes, that can, like Star Wars, map pretty easily to the Hero's Journey. Heck, a trip to the store can map to the Hero's Journey. So, yeah, you could run a compelling Hero's Journey kind of story in any version of D&D, or any other system, for that matter. But it won't be the experience Mearls describes. It might be a great - or a better - experience, but it'll be a different one.
Also, the experience can't be uncoupled from the person experiencing it. Mike's comments resonated with me, because, I'm guessing, we shared comparable experiences with that edition of the game, back at the height of the fad years. Maybe we have some other things in common, too, that shade that experience, the memory of it and the quality of the
nostalgiaemotions that revisiting it evokes?
So is it /more/ than the system? Sure. But it's not /not/ the system.