Bun folks routinely contribute to WebKit, and bun itself is an incredibly impressive project, so I don't think they're lacking expertise

I think they do. Building bun is a complex task and engineers who can do that should also be able to figure out how to help out with a compiler. It is just a matter of immersing yourself in the code and be willing to put in the hours and hard work. Sure, they may not be able to help out with designing the type resolution but there is other work which needs to be done that any skilled engineer can do.