I wonder if his coordination in other areas has improved as a side effect?

I do some light juggling adjacent activity, and I think it's absolutely improved coordination in other ways. When things randomly fall unexpectedly now I feel like my hands snap out and catch them so much more deftly than before. And with a crawling baby in my life now, I'm often catching them as well.

Greatest pride was catching a tumbling beer at a party, and getting called spider-man.

I had the same experience.

Getting good at juggling makes things fall down noticeably slower. But despite getting really good at juggling, I haven't observed any other super powers.

I know mine sure did! As a kid I was totally uncoordinated, always the last one to be picked for any sports teams. But I definitely noticed the difference when I picked up juggling as a young adult.