Having seen parts of the speed run, you start as an Origin character and do a lot of jumping (keeping this effectively spoiler free). I don't think they're using out of bounds, but they do "complete" the game before act 3 even starts. It's an "Any %" run, which essentially is a run that takes you to the credits even if it's not the end of the actual game. Apparently there is a way to trigger that state earlier in the game. Having pased that point, it makes sense what it is and why the game allows it. But it's not something you'd likely run into on your first playthrough.

I'm sure there will be other categories added later, likely "Any % Act 3" or an any % category with other requirements, like specific boss kills or end conditions.