FNA is a reimplementation of the old Microsoft XNA framework. I think the appeal of FNA is mostly to keep old games alive, rather than as a platform for new ones, so it feels a bit strange to see it get as much funding as Godot here. Terraria itself is developed in XNA (and presumably uses FNA now) though, so it could be pure self-interest (which isn't a bad thing).
Yes, it would be odd for Terraria to fund solely something they don't use even if it's more relevant for replacing Unity.
I was actually involved in an engineering software package that used XNA. It was resurrected a few years ago after close to a decade on ice. We were sure glad that FNA existed because Microsoft had dropped XNA like a hot potato. Yeah, I wouldn't trust them again either.
