This actually uses futexes indirectly, in my understanding from the post, and it's not the most awful thing for Linux.
It's possible that Linux futexes are not a bad idea (but it's also not clearly a huge improvement), but s_lock does okay and has existed an awfully long time (with iteration), so there's some inertia there.
Also, I think I oversimplified the parent's post, he could have simply meant that something about s_lock is not very good, as opposed to "self-rolled user space spinlocks? That's ridiculous!" (as a straw man). It's possible s_lock could be improved, however, it was fine before and doesn't clearly seem at fault right now. I think the suggestion of having to manhandle the platform-specific scheduler also seems excessive. There may be a better solution to serve everyone; somehow I can't see PostgreSQL's spinlocks being one-of-a-kind in this pathology, but I haven't attempted to prove that.