They are legally obligated to show which part of the video is an ad (and contractually obligated to have a clickable link), which always leaves ad blockers a way to correlate and remove those segments though (essentially skipping forward during the ad, then lying to the backend when asking for additional segments as if the user had skipped through the video after the ad was over).

On Twitch they managed to outplay even uBlock, because the streaming is realtime and if you skip the ad segments, there’s no data to fall back to and the backend won’t send you the regular segments until the ad break is over (from what I understand). So at best you get a waiting screen instead of an ad.

However I’m not sure if it would make (financial) sense to apply a similar strategy on YouTube, as that would require preventing buffering the video until the ads have stopped playing (and wouldn’t work at all for midroll ads since the video has already been buffered at that point). Not only would this be expensive to do in the backend, but it would likely cause disproportionate buffering on low-end connections which couldn’t start loading the video while the ad is playing.