It's probably exactly what it looks like -- some guy riding alongside.
Probably not very clever of the guy, but loads of the course (even up to one block before the finish) was just wide open public road. Nothing to stop anyone from hangin' out doing whatever along the roadway.
You could even cross the race course like just meters in front of the finish line all day, in between rider arrivals. You'd go up to a volunteer, ask if it was cool to cross, they'd look back up the course to make sure nobody was coming, and you could walk right across. Also, you could hang out just beyond the finish line, and walk right up to riders as they crossed if you wanted a chat or a snapshot. Pretty low-key and wide open all around.
What a cool day!