But they will have to alter othet elements on the page. For example, scrubbing. It will either have to be paused at one specific timestamp while the ad is playing or the ad would have to be incorporated into the length of the video.
In either case, it is detectable.
The video chunks hash can be calculated and then blocked, in a crowdsourced way like with sponsorblock (but way more effective, because it will cover all videos)