Prospective supervisor wants me to do irrelevant videos as a condition for supervision. Can I say yes when I mean no?
No, this is not okay.
If you lie your way into a position you are not suited for, you will almost certainly pay for it down the line.
That's a very general rule, and I think it applies here. If you say you are willing to do X while interviewing for a PhD position and, after getting the position, you refuse to do X, then you are going to have soured the relationship with your advisor. That is a very important relationship. If you say no in the interview and you don't get the position because of it, then it's probably for the best.