It's a 2D sprite based fighting game engine that lets you import characters and stages from literally any fighter ever made (even some non-2D ones too!).
Sadly the engine is closed source and is kind of abandonware but it's kept alive by fans over the years.
There's so much you can do with it. Sure a lot of it is jank, but thanks to fans it's a very reliable way to play sprite based fighting games on PC outside of emulation.
Is this the engine salty bet uses? It sounds familiar.
Yep same one I believe, most likely it's modded though for their twitch channel.