It only matters in a positive sense. Being written in TypeScript signals that it uses modern features that makes the code easier to read, that it doesn't use to

It only matters in a positive sense. Being written in TypeScript signals that it uses modern features that makes the code easier to read, that it doesn't use too many strange dynamic features, and of course that there are type definitions available for other TypeScript users. Other users can use it like they would use any JavaScript library.