Sure, I knew that. Keith's busy to nervous/playful original bass track is legendary.
You can hear it's instrumentally mimed - there are even a couple of moments where Mick is not near the mike and having his mouth closed while singing is going on. He sang parts of it live, but not all.
But seeing Bill with a not short- or medium-scale bass and then a Ric of all basses is a seldom sight. Maybe it was just a studio prop or maybe he felt inclined to give it a try as the less-than-long-scale of a Ric was attractive to him, who knows.