If a library was written in typescript, theb the users of the library wouldn’t be forced to use Typescript also, right? Does it really matter if the library is written in typescript or not?
Perhaps writing it is a bigger ask--a twenty minute session of reading and poking at the language in VSCode did it for me, but maybe I'm secretly some kind of TypeScript savant--but this just plain should not be a problem for anybody who's up on modern JS.
Typescript may be different and may be better, but there is a lot of rightly justified hesitation to adopting it.
For awhile, React/Redux was really kind of gross with TypeScript, but even that's mostly hammered out. (React's new context API did a lot to reduce my use of Redux in general, and works pretty comfortably with TypeScript too.)