Prospective supervisor wants me to do irrelevant videos as a condition for supervision. Can I say yes when I mean no? This question is the reverse of Dealing with a PhD student reneging on an agreement to appear in social media, in which the student had agreed during interviewing to participate in social media outreach but refused to do so when hired. The top answers in the linked question sided with the student. 

Suppose I'm a prospective PhD student looking for potential supervisors. I'm attracted to one professor in particular for both academic (e.g. matching research interests) and non-academic reasons (e.g. solves my two-body problem). The only problem is, he wants me to make social media videos to boost the profile of his research group. I think this is unreasonable, since doing this isn't part of the requirements for a PhD. I know I'm unlikely to enjoy doing these things as well, and would rather concentrate on my PhD.

The professor wants me to confirm both verbally and in writing that I'm willing to make these promotional videos. It's likely that if I say no, he will decline to supervise me. Is it OK to say yes, and then try to get out of making the videos?
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.