Ultimate was added to Garuda event about halfway through. It seems that future advent type events start with ultimate available already.

Note: When they updated the game to include Ultimate difficulty for Garuda, they also added a new SSR Kamihime, Fudou Myou-ou (aka Acala). For you re-rolling nuts out there, I have no idea if Fudou Myou-ou's addition cuts short Amaterasu/Agni/Hestia's new release rate. A new SSR eidolon was also added, Ifrit.