It doesn't. You might be thinking of libraries that you wrote, not packages from e.g. npm, which are distributed as JavaScript + type definition files not as TypeScript code.
It was added like 3 years ago which was probably a bit too late, not even sure why it's not the default. (File size?)
IDEA adds its own analysis on top of that provided by the language server.
Works on JS + every variant of type definitions I've ever seen, among many other things (not only programming languages, but also database objects, etc).
IDEA implements its own analysis and doesn't use tsserver at all. Its semantics diverge in subtle ways, I believe in both directions (some code that tsserver considers valid IDEA will consider invalid and vice versa).