That also fits with the rest of the possession timeline we have.
Ultimecia rather consistently keeps searching for information on stuff Edea obviously would already know the whole time, as if despite possessing Edea she has none of the knowledge of her current host.
That was the biggest bit of foreshadowing going on for this development: All the stuff that was being sought out by Ultimecia's extreme methods was things that Edea would probably have known just from how involved she was with them.
Even after clarifying the text myself, I'm still not sure how it all works
exactly. In itself, the idea that Ultimecia possessing Edea's body just controls the body without any access to the mind makes some sense, and fits with what we know of Sorceress Edea's actions when possessed, viz does not know what Edea should already know.
But then almost immediately after that line, Edea talks about Sorceress Adel, and has this line:
Edea: "Sorceress Adel is the type who will not hesitate to use her powers for her own selfish desires. Should Sorceress Ultimecia from the future bequeath her anger and hatred unto Adel, their power could be unimaginable…"
This is mostly translated accurately, except for the part saying "bequeath her anger and hatred"; in the Japanese text it's "power and anger". (And "unimaginable" is more "fearsome"/"dreadful", but same idea.)
This line implies if Ultimecia possesses Adel, it will be
Adel in charge of the combined consciousness. Ultimecia's contribution to the possession is "power and anger", which will apparently be horrifying beyond just Adel's own power. But it's Adel's motivations and personality that will be the source of the problem, rather than Ultimecia's goals.
I can only speculate that Edea's method of inviting Ultimecia to possess her is somehow different or special, leading to Edea's personality becoming unavailable. If Ultimecia possesses Adel in her own way, though, Adel will still be fully conscious and able to access Ultimecia's motivations, and vice versa. However, this is not stated in the text, possibly because Squall is mashing the Skip Cutscene button.
