Disingenious. A better way to say is that their particular socket implementation was inferior to their new proprietary API. There is nothing with socket paradigm as such that gives it inherent performance disadvantage.
To clarify, the whole networking stack isn't userspace. They still have the classical BSD networking stack. I think the userspace networking stack is used automatically by system libraries like URLSessions and this new Network.framework library. They introduced their userspace networking stack during last year's WWDC: https://developer.apple.com/videos/play/wwdc2017/707
It's an interesting read. I'm not finished with the video, so I don't know if this is the way that Apple went.