vnd has already verified that the context tag is valid before this gets called, and we only set the context tag private data to non-null for indirect clients. Mesa happens to be buggy and doesn't send MakeCurrent requests nearly as much as it should for direct contexts, but if you fix that, then unbinding a direct context would fail here with GLXBadContextTag.
Sadly Mesa will still need to carry a workaround here for broken servers, but we should still fix the server.