I seriously doubt that SE didn't realize they were making a spell name Latin...Actually I find it funny that you even perplexed such a notion. One of the major reasons why I love FF games is that EVERY unique spell name, EVERY monster name, even their character names have a real life history in the classics. And they do it with a cut of elegance even, basically what I mean if Joe-Schmo MMO had an equivalent spell it'd probably be called 'Big Bang' or 'Mega-Explosion' but FF took the time to find a Latin word and called it Ultima.

Also I wasn't trying to say what Comet was in XI. I was merely stating that TRADITIONALLY it was a spell reserved for Time Mage and was considered Time Magic not Black/Dark/Elemental Magic, as an old-school FFer; I'm pretty certain on this claim. And that Ultima (while learned from Black mage-esque characters in the FF series) was a NON-ELEMENTAL magic spell (Traditionally speaking).

And the reasoning why to make it a schema is:
Adloqium
Animus Augeo
Animus Minuo
Tabla Rasa
Modus Veritas
Ultima

Are all Latin words and already fit as the spell names. And being that in terms of XI's lore that Black Mage's are the masters of ELEMENTAL magic then it makes no more sense to give them a NON-ELEMENTAL magic spell...Does that make it a little more clear as to the reasoning?