Also possible that she's taken it up as an identity, or that it's a title, or that this is an alternate universe/timeline version of the characters, wherein the equivalent to our Y'shtola is named Matoya - all of those, plus the "What would Matoya say?" option, feel like they are much more likely to be what is going on than Matoya suddenly deciding to leave her cave and go out and possess the empty shell of her own protege...especially for an individual that -last I checked - does not have the Echo and is not a Resonant.