Itd say its probably legit because Pew isnt contesting the info at least (he's even making his own theories on twitter) and these guys clearly have scans.
Imo, if the image is real, then its just Shanks to keep things simple
there are a lot of ways this angle could go.
if its just shanks, oda could continue setting up the possibility of two of them so that the audience can accept both the "good" and "bad" shanks personas since they're separate people, and then reveal they really are actually the same person and have it possibly hit harder. he could also shut down the double angle early and just build tension by having it revealed long before the characters find out or can do anything about it, but that's rarely his style.
having there actually be another Shanks also opens up a lot of possibilities, but i feel they're overall more "hack". like tricking Luffy or anyone else with the fake Shanks, having Blackbeard kill Shanks but it's not the real one so people can eat their cake and have it too, having Blackbeard kill the real Shanks and the govt. replace him with the Holy Knight one, having a big overblown metal gear solid twin brother thing, etc