Same here. Open a JS project in a Jetbrains IDE and things start going red all over the place. Asking the JS devs about it, they're like "oh we didn't us TS on that file" or "oh yeah we know about that, just ignore it". And the best one "oh yeah, that's a bug, we haven't finished that part yet."

Honestly happens with python devs too when you open their project or code in a "real" IDE that checks for all these things.