Okay, let's be SUPER GENEROUS and say there were a couple of dozen nutjobs armed with pepper spray and signs who thought they were going to bust in the Capitol and prevent Joe Biden from being certified as the winner of the election.

What was their probability of success?

Very poor. Which wouldn't affect whether this was an insurrection or not - most insurrections have poor to vanishingly low odds of success.

But....not zero. Had Pence been killed, or scared enough to change his position, then they would have been successful. As it was, they did manage to prevent Joe Biden from being certified as the winner of the election for at least a couple of hours longer than would have happened. Had they managed to stretch that out from hours to a day or two (unlikely but not insanely so), it would have created enough space for something to happen that might have led Pence (or enough more Congresscritters) to jam up the process. I think that was what the non-nutjobs that were taking this very seriously were aiming for - to create enough chaos on Jan. 6 so that the certification stretched outside of "normal" processes, creating an opening for other players in the process to achieve goals that would have been foreclosed if everything had just gone according to rote.

Unlikely? Sure. Any insurrection against the U.S. government is unlikely to succeed. But storming the U.S. Capitol with all of the elected members of the federal government (save the President) inside of it while doing an essential act is probably the best of all scenarios for interfering with the transition of power.

I don't think there was much chance of that happening, but not zero chance.