Reality isn't black and white. It isn't either "you welcome mods" or you're "hostile towards mods". There's a lot of grey area, and ignoring that gets you nowhere in life.
Frontier obviously isn't hostile towards the concept of mods, there's literally a Thememaker's Toolkit that was released at the end of Planet Coaster that does exactly the thing you described (minus the paying part, but hey, that's not that common either way).
That being said, they simply have no official support for it (right now) and that's totally fine, that's how it goes with loads of other games. Endorsing them means a whole other legal stance they have to take and brings in a lot of extra work (aka, doubling down on checking whether a bug is caused by the game or a mod and ensuring that every mod ever doesn't break which is absolutely ludicrous to begin with) and they simply chose not to do so. So to avoid confusion, they don't allow it here on the forums and that's fine.
But even without official support, the devs have literally fixed issues already that helped modding, by removing the bug that if an animal isn't available in your game but still in the trade center the game would crash, or removing the species limit that was originally 256 animals. These changes didn't have to be done really for people who don't mod, but they decided to do it anyway.
Heck, if they were really so hostile towards mods they wouldn't let the content creators they work with use mods, and yet they still do. It's completely fine, nobody has ever mentioned anything about it towards them ever.
So no. They're not hostile towards mods at all, they just condone them in stead of providing official support. And that is completely fine, as is with so many other games. No software company ever, regular or game, owes you modding. They simply don't.