Same gig; Derek is called up
Yeah, he is plugged in to an amp, and you can see Derek motion to the person running monitors and point to them twice. I don't think he was happy with the tone he was getting from that amp, and had no time to make adjustments, but made the most of it and enjoyed the moment.
I saw a couple comments that suggest that his band was playing that same night and he went over to sit in with Billy during a set break, which is pretty cool.