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.

Hard No.

This isn't only unethical, it's also a really bad idea in practice.

Unethical: Every PhD position I have ever heard of requires you to do things that aren't part of the requirements to get a PhD (teach, do project work, help your supervisor with reviews, and so on). This is why you get a stipend or salary. Your prospective supervisor communicates very clearly what they expect from this position. It is not your place to decide that you would rather interpret the position differently. You are of course free to disagree with the assessment that this is important or fruitful, but then you need to raise your concerns equally clearly and upfront. Lying about it isn't ethical.

Really bad idea: You have been around this forum for a while. You must have noticed how all the truly terrible PhD experiences that people have start with a student having, for one reason or another, a bad personal relationship with their supervisor. Your prospective supervisor clearly feels very strongly about these videos, and most people care about not being lied to their face. If you now provide, in writing nonetheless, that you will help with these videos and then refuse to do so, you are running a serious risk of fundamentally breaking your personal relationship to the supervisor before it even gets a chance to develop. A few videos are surely not worth this risk.

If you decide to take this position, I suggest doing so with the mindset that you will in fact work on these videos. You don't need to be excited about it, and nobody can force you to do the best job in the world on them. Further, there is always a chance that your supervisor changes their mind on the subject, but I strongly suggest you don't take the position now with the full intend to weasel out of this task.

Compromise

You aren't interested in social media (albeit, you should perhaps give it a try) and you don't want to devote time to it, because it will detract from other activities, such as writing papers. But, social media is a formal requirement of supervision (in this instance). So, compromise:

Agree to participate in the production of social media videos, on the basis that you'll provide background material, be interviewed, etc., rather than the production aspects. Perhaps suggest that these aspects can be delegated to an undergraduate student (who could be paid), maybe a student in media.

Also, remember that dissemination is part of research, so perhaps also offer:

To keep an open mind and be willing to take a more active role in the future.

Thereby keeping your future options open.

Do not agree to something you won't do