I feel like Donkey Kong 64 is infamous for its density moreso than sheer length exactly. Maybe for its time it was kind of long? But nowadays it's hardly special in that department, games being offensively long is basically celebrated. The only reason DK64's, uh, girth, is still notable is that it takes the collectathon gameplay loop of "collect things to open things to collect more things" so far into absurdity that it starts to resemble a grindy mobile game with more currencies than its AI-generated art has fingers, in a time before horse armor was a twinkle in Bethesda's eye. And as far as I'm aware, no other platformer to this day, whether it's a giant modernized bloatfest or an actual mobile game, has yet come close to surpassing it at that.
...And this is also silly because yeah, if you aren't constantly going back and switching characters, the number one thing people complain about to this day with this game, it makes the game go a hell of a lot faster. You don't necessarily need to have foreknowledge of the levels, but it requires you to go over the whole thing and remember all the things you passed and then plan who to switch to next to cut down on unneeded trips, and most people clearly do not naturally approach it like that.
Not to mention that this is a TAS of Donkey Kong 64. I haven't watched it, but I can guarantee that what this run consists of will include very little of what would be recognizable as "playing Donkey Kong 64". This is a game where you can clip through walls at will by throwing some oranges to create lag, or swim through the ground if you just keep trying for a few seconds. The shit that is possible beyond human limitations is going to be downright stupid.