PostgreSQL allows function overloading, which means that there can be multiple functions with same name differing in argument types. So, the syntax has to include the input parameters or their types at least.
"local_routine_name" and "remote_routine_spec" were my own paraphrasings of what the spec implies. I'm nearly certain that the local routine name, which the spec says is just an identifier, cannot have a parameter spec on it, which leaves only one other place to define it, remote_routine_spec, which wasn't defined at all. I _suppose_ parameter definitions could be pushed into options, but that'd be ugly.