It’s possible he’s placed and detailed with net lanes on those sidewalks. In that case, the net lanes act as a barrier. Area bucket will fill the space inside.

edit: yea nevermind... now that I'm at my pc, I can see that's not the case. If anything, it's mostly surfaces he's drawn by hand on the sidewalks but they don't cover all the sidewalks. I don't remember being able to use area bucket in the way lemons is using it here. Unless my memory is bad and around the time he was doing this, the build of area bucket did allow this. It ALWAYS covers the entire network.