I'd thought that Nabriales pretty much confirms it when stealing Tupsimati and making off with Minfilia...

Nabriales - "Unlike the others, I am not given to waiting. I shall take that staff and bring about the next Rejoining."
Minfilia - "...Rejoining? Then it was your doing! The Isle of Val, the scholars - all of it!"

And...he never acknowledges it, one way or the other. He just does it little teleport shift thing behind her again and starts on about taking the staff.

So oddly enough, the other knowledge about it being an Ultima-like spell is more confirmation than the direct accusation, unless we're supposed to take Minfilia's conclusion as the right one (which I'm guessing it is, but from an in-universe perspective it's still a potentially fallible character reaching a potentially incorrect conclusion).