Yeah, given the state of development the game should be in, most narrative tasks should have been locked on for a few months now. So i don't really think the firing of Mitsoda will have much impact on the story if at all.
Still doesn't change the fact it seem rather shitty to fire him like that. But maybe there's a good reason, maybe he was a bothering people in the studio for some reason and they felt it was safer to remove him. Things happen. I mean, i think Ubisoft more or less did the same thing to the creator of For Honor, ditching him a few months before release. The game still was fine in the end.