With a 2d sidescroller the streaming doesn't have to be too crazy - not like a big open world game. The fixed transitions like in the clip cut from NNT make it a lot simpler too, by introducing a hard barrier for the player and giving you a fixed amount of time to load. You can maybe get away with unloading parts of the level as you go - more so in AE which introduced slam doors that can permanently stop you backtracking - and then as long as the transition is loaded you have that time to just load the first bit of the next section, and ensure that nobody can get ahead of the rest of the level loading. Again to be AE specific, if I was working with a system like that for the game I'd probably make good use of the wheels that take a certain amount of time to turn in order to gate progress to loading. Maybe some wheels even take a bit longer than the others
I can't speak for performance coding that above Unity's systems though. The fixed transition-tunnels and side on camera perspective make it a simpler problem either way.
Also I'm no expert or anything. I have been thinking about streaming level formats a lot lately though.