"sealed deep within the tower" isn't referring to Horutoto or Pso'Xja, it's referring to Heavens Tower.
Beneath Heavens Tower is Toraimarai Canal. The area that's sealed in there is behind the "Marble Door" that we accessed in Windurst 7-1 (The Sixth Ministry). Behind that door is an exit port directly into Heavens Tower. The Sixth Ministry is the Animastery in case you forgot. It deals with the creation of Cardians. It is definitely what's being talked about.
https://ffxiclopedia.fandom.com/wiki/The_Sixth_Ministry/Plot_Details
Normally we pass the Marble Door with the Optistery Ring KI, but that's consumed in the Windy missions and received from Tosuka-Porika at the Optistery - who isn't currently giving out new ones.
So that's what they're talking about, but does it have any bearing on finding the Dream Flower Petal? Might we need to trade something to that Marble Door? Might the drop be hanging around in Toraimarai?
I don't think so. Having seen the dialogue that follows handing it in above, I reckon the Dream Flower Petal was merely used by the Yagudo. Hence I'm off to Castle Oztroja on a murder spree.