Im pretty confused myself.
Edea tells us that she got her powers from a dying sorceress when she was young.
And thats exactly what happens in the end. Ultimecia dies, and she gives her powers to Edea.
If it wasnt for Squall and the others, that would've happened.

You can argue that the Garden and SeeD's never would've existed if it wasnt for Squall telling Edea in the end of the game...so it kinda loops or whatever..

I dont know, its just damn confusing