Why does Joe Saylor sing before Patrick Bartley comes in?

I stumbled upon this video from George Collier’s Youtube channel and I’ve now listened to this a countless number of times. Since it’s so popular on Youtube, there are several commentaries and reactions to this, even one by Patrick Bartley himself.

But I don’t think I’ve seen anyone address why Joe Saylor starts singing at the very end of his drum solo. The video link above should take you to 12:50 in the video. He’s definitely singing part of the lick Patrick is about to play.

So is he singing because Patrick missed it? Was Joe early?

I see Patrick looking back to see where the bassist and trumpeter are so it feels like a cue was missed somewhere, but that’s what this makes this so fascinating. The band continues seamlessly and song continues and everything still sounds great. How do professional jazz musicians do this?