It was Beryl in Olympus right after he betrayed Kirschtaria.
Spoiler:
Obviously this excludes Olga (had yet to descend) and all the dead characters. And yeah I doubt he knew much about Kukulkan.
However there's a second statement at the start of the part 2 of Avalon le Fae:
Spoiler:
The craziest thing is that at this point they didn't even know about any of the other Rhongomyniads apart from the one they had seen.
It's pretty consistent since her Rhongo was said to surpass Kirschtaria's magecraft (who in turn is strong enough to be respected from Zeus & is able to easily one-shot Demeter), and would have destroyed the entirety of Olympus was it not for Wodime barely managing to block it (his magecraft was starting to show cracks by the end of the cutscene).
One point in Zeus' favor would be that he had only unlocked 35% of his limiters and never really went all out on us, but at least the one we've fought should be inferior to Morgan.