TypeScript has nothing to do with C#, apart from its creator. The structural type system with explicit nulls, sum types and very powerful generics makes it feel much closer to the ML family of languages than C#, despite using the C-like JS syntax. The teams I've used it in have never really used it to build complex OOP hierarchies or even used classes for anything other than React components, though there are probably many who do.
Both are true. The original TypeScript v1 was a lot more like C#-in-JS and was very class-oriented, however over the years the type system has grown into a mature ML-like structural type system as a side effect of trying to type the wide variety of dynamic JS out there.
Anybody who last looked at TypeScript a few years ago and dismissed it needs to have another look.
