Do they use a third party support lib?

If it's possible to support use cases with a third party lib, then I'm less concerned - provided that that lib also works with musl. Given the nature of ucontext, that may not be so. A musl native solution would be optimal for performance - and performance is a common reason for going down this route. It allows for far greater scale in certain server designs then either thread-per-connection or a thread-pool can do.