It's not as easy to just refactor when it comes to UI oftentimes because you are often writing completely new UX.
It was probably bad wording on his part when he said “rewriting from C# to Angular”. If he meant, “we created an API in C# and decoupled the MVC rendering from the business logic and rewrote the front end in Angular”, that’s different. Since he didn’t mention rewriting the back end, I can’t assume they rewrote the backend.