yes. But then they have different problems. Now it is the ad company who is responsible to serve the ads and the personalization comes from there. This is achieved by the client directly "asking" the ad company for ads. If they want the ads to come from the same stream this means that the customer identity is passed to youtube, then youtube requests the ads in behalf of the client, and then serves them mixed in the video stream. I'm not a lawyer but I think that this causes different legal problems for youtube on the part that they will need to ask the ads on behalf of someone else.

Also apart from that, technically, the part of the video that is an ad, will be associated with a call-to-action URL and an overlay on top of the video, since they need that by clicking on the video it will go to a the ad's call-to-action instead of just pausing the video. This will still make them detectable