In this case, not really. TypeScript can't be sound because there is zero runtime type safety in JS. That you are able to do `as unknown as T` makes TypeScript unsound, but it's also an escape hatch often needed to interact with JavaScript's dynamic typing.
It's never needed, it's just often convenient for something quick and dirty. You can always write a guard to accomplish the same thing—roll your own runtime safety. If you want to avoid doing it manually there's Zod. It's not that much different than writing a binding for a C library in another language in that you're manually enforcing the constraints of your app at the boundaries.
