Oh, I don't know. I don't know anything really, which is why I posed it to the OP as to why he/she believes it's one of Arnold's backstories. I wasn't sure if maybe I missed something. We do know that Teddy's new backstory is rooted in truth, according to Ford, and it seems to tell a tale of two friends who ended up as enemies, and the way it's been shown so far (even if the memories are fake) that it was Wyatt who went nuts, not Teddy. If that's true, then Teddy is playing "Ford" in this narrative. But...we know Teddy is an unreliable as a narrator (including what he tells MiB about Wyatt), so I could see a twist in which Teddy is the one that initiated and led the massacre, and is the "Arnold" figure, not Wyatt.
What do you think?