Video Proof: Which Cup Will Fill Up First?

This puzzle has been shared on social media a lot recently. I gave it a good look and I thought....duh...that is easy. (eyes rolling) However, not everyone was getting the same answer as I did.

There's a youtube video solving the puzzle...BOOM! I picked 4, Bobby picked 4....BUT THE 3 CUP IS BLOCKED!!!! WHAT?! I didn't see that.