I don't really care about full game speedruns of JJR but:

Adventure Mode (difficulty) and Speedrun Mode (difficulty) are the category names (I imagine) because of the leaderboards at the end of the game are in those game modes (and those specific difficulties).

I highly doubt someone would do an "All Stars" run over a 100% run which is also not very desirable to do due to the way it works (you essentially have to replay every level at least twice).

"This adds a whole new layer of gameplay for speedruns." Not really. 100% would be a joke of a speedrun (Have you actually tried doing 100% game completion? You have to intentionally slow pace a lot of the stuff.)

Keep in mind, not everyone needs to use speedrun.com because there's already in game leaderboards to compete with. The only thing tracked in the ingame leaderboards is how fast you beat the game, which drives all the competition to be in an any% format.

JJR doesn't really need a speedrun.com profile due to the in game leaderboards, but it's used as a way to share replays of your times that are on the leaderboards.