I think it was less dictating, and more of being aware beforehand. The shows of TV era all have a pretty distinctive way they're cut, to have breaking points for ads. It gets especially noticeable today, when you're most likely to be re-watching them on an ad-break-free streaming service.

On the other hand, apparently there is a way for the YouTube creators to dictate where ad breaks will happen. I've seen at least two videos in which the person speaking said "and then YouTube will show you an ad right about... NOW"; lo and behold, an ad shows perfectly timed to when the speaker announces it.