C#'s embeddability/interop story is as good or better than Golang's. The primary reason they chose not to use it was that C# was seen as too verbose compared to the TS original source. In particular TS used a lot of top level functions and bare/anonymous data types and while C# has picked up top level function support it's still not "idiomatic" and while C# has a long standing anonymous data type support it's not very "ergonomic", certainly not compared to TS structural typing. Golang's traits are a bit more ergonomic by comparison, unfortunately.

I also still think C# would have been a better long term option for maintainability, but I'm not on the Typescript team and didn't get a vote, and I can understand their methodology even if I don't agree with it.