But someday, she promises, she will return to us in triumph.
That specific quote doesn't refer to any form of return. She claims she will one day return the favour (regarding the WoL having always been there for her). This is not the same as Minfilia saying she promises to return. She does imply that elsewhere during that same cutscene/moment - that she will try to find a way to return, but that in itself is not a favour to the WoL - what that is remains to be seen, and likely has something to do with diverting the cataclysm and all that (again, assuming this all isn't some light/dark ruse/ploy).

We can see now that she does return in one way or another (unless this is some sort of light or dark shenanigans), but the favour you refer to remains to be seen.