It's not "for certain" certain, but I'd say it's sure enough for us to run with. Given that the Eleventh Doctor died on Trenzalore due to not having any more regenerations (as mentioned in The Name of the Doctor and The Time of the Doctor) it seems like speculating a little too much to really consider her a 6B incarnation when Eleventh was the last of the Doctor's first regeneration cycle.
Obviously Chibnall could probably find some way of doing that, though.