How to "block" ads: Refresh the video multiple times. They will show a few different ads and then give up. It even works on console YT apps which have more ads.
I guess it affects impressions? I figured they would have fixed that, but this still works so whatever.
It's a user retention thing: if a user is reloading a video repeatedly while they're trying to serve an ad, there's (probably) something wrong with the ad server, so the best business move is to send them to the content they came to see, unmonetized, so that they will continue to use the platform in the future where you can monetize their future visits.