feval() is not a keyword, so the parser still has to do name resolution on it. The difference is that the name resolution would determine that it is a function, potentially allowing earlier optimization than the case where it is not known at parse time whether a name passed in is an array or a function handle.
One thing I do not know is whether if you were to use an argument block to enforce that the parameter is a function handle, then would the parser be able to reason about that to be able to hypothetically do the same kind of optimization described above? ... Though surely the cost of deriving that kind of information from an argument block would be higher than the cost of deriving it from doing name resolution.
