This is smart. It could be used to check other TikTok videos by the candidate. Like who would create a new blank TikTok account to upload that video? I'm not saying that the company does it, however I see this as a possibility.
Actually, if that was the case, I'd have an idea. Make a TikTok right during the interview together with the interviewer, saying exactly what you are doing right now and for which company you are interviewing. But don't actually upload that video, only wait for the reaction. Then ask if you may upload that video. If you may not upload it, ask them why. Use their answer as an explanation, why you won't give them your TikTok account.
Actually, if that was the case, I'd have an idea. Make a TikTok right during the interview together with the interviewer, saying exactly what you are doing right now and for which company you are interviewing. But don't actually upload that video, only wait for the reaction. Then ask if you may upload that video. If you may not upload it, ask them why. Use their answer as an explanation, why you won't give them your TikTok account.