YouTube’s TOS would be the most critical place to begin in terms of evaluating legal options. To file a “DMCA” (not really DMCA but YT’s proprietary version of it) claimants generally have to create an account and agree to the TOS. So it may bind both parties (the YTer and the abusive DMCA claimant). That might limit legal options for anti-SLAPP, tortious interference, etc.
But without either significant legal expertise or someone finding some particularly relevant case law, it seems like a nuanced enough domain that no one’s lay “legal” opinion would be particularly illuminating.