Considering his previous statement was literally "Anakin Skywalker was weak. I destroyed him." and the tone which he took in saying so, I think we can safely say that the Anakin persona had not surfaced.
More likely, we rarely find those with lightsabers are force abilities to not be associated with Jedi or Sith (although admittedly, such is more common at the end of Clone Wars and throughout Rebels). Being that Ahsoka is actively working with the Jedi protagonists, and given her past as a Jedi, it may not have been all that odd for Vader to assume she had gone back to following the Jedi way (after all, in a sense she did in principle behave and function as a Jedi would, even though in name she makes it clear that she does not identify herself as such).
Because of this, and given his past knowledge her character, it makes sense for him to be surprised that she would go so far as to seek revenge, something very "anti-Jedi" of her to do. Ultimately, it also serves to show that just as he changed since their last encounter, so had she.
EDIT:
It's also possible that it may have been a lead-in to turning her to the dark side. Pointing out ways that she had already started on the path, much like he did with Luke, could have been the start of his attempt to convert her. But after her response and readying for battle, it became clear that the time to continue such banter had passed.