Wait a minute...is this a plot-hole (Austerities of Wind and Main Scenario-related)?

[Anonymoose]

Don't forget the biggest hint the game gives you: you can only summon a primal you were there to defeat.

If I remember accurately, and infer corrrectly:

Defeating a primal causes its aetherial form to shatter; it rapidly dissolves back into mist and returns to the Land (or, more vaguely, to the natural order). One who has defeated a primal has themselves necessarily been bathed in this aetherial dispersion, and is forever host to a trace of its essence. Egi, therefore, appear to be summoned, at least in part, from within.

The best theory I've got at the moment is that the trace of Garuda's essence left on the summoner is harnessed to "program" the gathered aether (as math would do for carbuncle). Ultima may have had Garuda, but it didn't have the traces of her essence left on us from her defeat.