I assume most of the minutes-long compile times they are running into are because of type checking. Moving types to comments wouldn't help that.

I don't konow why they aren't separating type checking and dev builds from each other, but there is just absolutely no way simple TS to modern JS (no compiling to old ES5) conversion would take that long for a 10k LOC project, it must be the type checking. I'm using TS through Babel and it has been a breeze for much larger projects than that. Babel just strips away the TS types and we do type checks via editor integration and a separate CI step. Can't recommend this setup enough.