Can you? If they track the time the user began watching the video, they can refuse to send you any more of the actual video until the duration of the ad is complete.
Buffering seems to have been chunked for a long time now, though. Ten years ago we could buffer an entire video. Now it’s limited to about 5% at a time. I guess that’s how the chunked HLS streams and such work, and YT could probably avoid serving certain chunks without some unique token received from watching the ad.