1) Nobody on nsm claims copyright for their work, whether or not that is legitimate under derivative works
2) There is no public statement on nsm asking for credit to be given, regardless of how "common sense" you think that may be. There probably should be.
3) There is thus no actual obligation besides the goodwill of angry kids for someone to provide credit
If someone is willing to give credit in videos going forward, that's more than you can ask for already. You get nothing out of and have no grounds to make demands in any way. Any further action on this will result in bans for user harassment.
It's clear that to avoid this kind of thing in the future, NSM should establish clear guidelines/rules as to how we want to deal with people reusing works. Those don't exist now, and may not for some time, so until they do, please be reasonable.