Earlier quoted context omitted.
I wish the article had gone into more detail about what the verification process is like.
Yeah, this. I don't see how they could possibly verify that this is actually an image of BtK when the face is so obscured and the provenance of the image so uncertain. Kagin's is being very cagey about it too: "Any qualified individual wishing to review the authentication dossier and discuss the potential acquisition of the Billy the Kid Croquet Match Tintype are welcome to contact..."
As well, the article mentioned that this is the first photo of him with some of his known gang members. The man to his left is quite clear, and therefore if they can identify him as an associate of Billy The Kid, perhaps that gave them more reason to suspect it was Billy.