It's an unsubstantiated rumor at this point so take it with a big grain of salt. But do we believe it?
I think it has a ring of truth to it. It would make sense for Tesla to upgrade the cameras at some point. Furthermore, other companies like Mobileye are using 12 cameras. So I could definitely see Tesla making a move to 13 cameras.
I've also noticed FSD Beta creep forward hesitantly a lot on the roads I drive which would suggest the current camera placement is not quite enough. So adding cameras would make sense IMO.