Arthur probably just don't care enough to tell the bbc they are wrong. It's more like his personal twitter, he never respond to fans or stuffs like that. I doubt he want it to become official.
There is just too many proof that it is Arthur twitter. It's easier to think that the bbc fucked up again.
It's his real twitter, not an impersonator. He posted picture of him, and his girlfriend also follow him and posted pictures on her twitter (my icon is one of the photo she posted). No way it's not him. It's just that his twitter go way back before he did doctor who so even the bbc don't know about it.
Yeah, that was what I always thought, it’s strange that the bbc posted that only yesterday.