I'm sure this was the only FIRST robot there -- I wasn't there; my friend Dave from work for some reason was there. (And I'm on team 1153, so I didn't have a reason to be there.) I can see why you missed it -- from the photo it looks like they were all the way on the north corner of the Plaza, no where near the T-stop to Quincy Market path. But it was a nice day.
Dave did tell me that the bricks were horrible to drive on, so they didn't. He also wondered who the ZZ-Top guy was.
You could go back today, but the Red Sox rally is there now. Lot's more people there, I bet.