Is there anyone who is in the beta program who has received this update? It's possible it's not being released to beta program phones. I'm in the beta and have not received, my brother has a 5x not in the program and has received the December patch.
Though I don't have a Nexus 5x, I DO have both a Nexus 6p and a Pixel C on the Beta program on 7.1.1, and NEITHER of them have received the December update, thus I'm figuring that because the last beta update is a NEWER release (has a later build date) than the official update, we must wait for Google to formulate a build that is newer than what we presently have before we can get it, otherwise it would be a downgrade. Perhaps they will either just eventually send a December security patch to us or update everyone to the same level in January.
This is my opinion, nothing more.