So. Just realised I didn't have the deviantArt on auto-sign in and haven't checked the inbox until today.
While this isn't technically my area/site of focus, I should have logged back in, not forgotten because RedBubble is the only thing I keep in mind. This part is my fault, sorry!
My guess is that the other artist whose accusation of plagiarism brought this case to our attention in the first place shot him a message at the same time as we did, and the guy conflated/confused the two messages as being both from us.
Shall we get back in contact with the artist? I can get her through a number of means, but if she saw the reply, chances are she's already dealt with whatever this person said to her. Additionally, I do recall the consensus with this case being that we were really only sending a licensing message because the artist had brought it to our attention for the plagiarism aspects. I'm not sure there's much else we can do here, unfortunately - unless we want to try to clarify to this person the difference between an official licensing message and a message from an individual artist unaffiliated with SCP staff?