I was at a tournament a couple of weeks ago where this question came up, and so long as there are no cones on the opponent’s mobile goal and your robot does not grab the goal on multiple sides (you can hook your arm over the goal or push it, but you can’t pick it up), you can move the goal out of the 5 point zone or just flip the mobile goal over to prevent the other team from scoring it.
It’s there, just the URL is truncated. It’s the second URL in the Pushing / gathering / hoarding section in Interaction with Opposing Mobile Goals.
(What? I’m not a creepy guy who keeps tabs on a question that I asked that is actually being referenced! Totally not )
But yeah, it is legal for the teams to push other stacks that aren’t in the 10 or 20 point zones. As stated above, the number of cones on the mobile goal doesn’t matter. I think that is just a wrong call on the ref.