That's only really true for the current GC design. I don't know if there is anything stopping someone from sharing memory via unsafe pointers and trying to manage that with a custom GC.
True, I don't know if anything's going to stop you, but in my uninformed opinion you'd probably be violating so many built-in assumptions that you'd have to perform invasive surgery on the runtime to get it to work. But I'm hardly an expert in this area.
