The narrator is unreliable, me thinks. Or he doesn’t know about Thamasa which is full of magic-using humans practising this supposedly lost art. And they’ve been subsisting since the ancient War of the Magi which spawned magic. Not 100% sure about the animals, but yeah, they probably didn’t even warrant mentioning in regards to the plot.
lol
And as for Thamasa, I think I remember that it was mentioned why they still are able to cast magic.
She clearly can use it before that time. You can make her use it….you know what I mean, d00ds!
Some humans like Celes have magic infused in them at birth. So my guess is that enemy humans were infused with magic.