we're explicitly bypassing win32 futexes for the normal path but still using them for the spinlock
I am not sure exactly what you are referring to here? I assume you are talking about that we WaitOnAddress when the LIST_LOCKED flag is set. In which case we are bypassing futex/WaitOnAddress in the Release path, but there is no point in avoiding it in the Acquire path, because, well, you have to block on the lock anyway.
And I did do a little testing on Windows, and yes Windows won't block on this bit even in the Acquire path. I don't know why, maybe to show off, as I really don't see a performance argument here. HOWEVER, this bit does prevent the linked from being modified on Windows, too. This would mean this MR is using this bit for the same purpose as Windows.