Static methods and classes are commonplace and a normal practice in C#, particularly as extension methods (which, quite often, you guessed it, act on data). There isn't that much difference between a type defining simple instance methods and defining extension methods for that type separately if we look at codebases which need to have specific logic grouped in a single multi-KLOC file like, apparently, TS compiler does. There are other issues you could argue about but I think it's more about perception here and structuring the code would've been the smallest issue when porting.

The ship has sailed so not much can be done at this point.