Just the way I described, I’m a software developer, it would be easy as hell.
Your browser requests the video, YouTube decides you have to watch an ad. The ad has 15 seconds unskipable. So the easiest thing they could do is not send you video data for 14 seconds (add a spare second for buffering to not piss off users who do watch ads).
Doesn’t matter if you call some endpoint, load the ad data, whatever. You’re not receiving any video for a while, which would piss people off enough to leave.