Hmm, if that's what she said, that's my mistake. I admit to watching the scenes at about 4am.
However, I do have to side with Fenral on the reason why. She seemed to want to guide us in the direction of banishing darkness. I believe how vague she was is the same thing as the short Echo scene with the WoDs from 3.1. It's SE leaking knowledge to the player slowly, teasing us with tidbits, but only just enough so that the universe's secrets aren't all revealed at once. And it comes off badly.
I'm actually wondering how this scene is intended to be interpreted, because I don't know if we're "supposed" to view it as a good thing to blindly follow your Goddess when she uses your friend as a mouthpiece, simply because she's your Goddess and "desperate."
Also, I'm of mind that it doesn't matter if Minfilia is alive, Hydaelyn did what Hydaelyn did, it can't really be defended. Do you know who is also alive?
Roundrox. And she's being literally mind raped upon the Illuminati's will. Are we going to suddenly forgive the Illuminati for that, even if they turn around and decide to shut down Alexander because we ask nicely?
It shocks me that anyone can even think Minfilia being alive makes Hydaelyn's actions suddenly "okay."
Edit: Referring mostly to Alisa with this.