I am wracking my brain trying to think of a way to defend against this

Why not think of a way to perform this attack? Despite almost zero charisma, the No Kings people organized a truly massive, non-violent protest. Who can you get to pull off mass market adbotage? Once you've got a credible mechanism, you've got a better idea of how to defend. Maybe go so far as to try to make adbotage happen. The first few attempts will fail or have problems, but produce enough ad clickers that you'll stress test the system.

for whatever reason you can't get it on the chrome addon store.

1. most people aren't running adblockers, so them turning them off and clicking on adds would add very little noise to the data

2. the ad companies aren't exactly honest with their numbers; remember the pivot to video? that was just an out-and-out fabrication. The ad companies would just lie to their clients about click-throughs, impressions, etc.