Sure, there can be comments in a header, but they aren't actually part of the API. They're just documentation that the compiler happens to allow you to put in the same file.

I'm not sure I'm with you on type names though. If the application being linked against the API was compiled using a header with different type names then the application would have to be rewritten to use the other type names. It would break source code interoperability. (And you can infer the type names from the ABI given the source and binary of an application compiled using the original header, without actually having the header.)