No, it means it does only half of the job. It can only take an input of fixed size and return an output of the same size. "True" hash functions take an input

No, it means it does only half of the job. It can only take an input of fixed size and return an output of the same size. "True" hash functions take an input buffer of an arbitrary size, and return an output of a fixed size. The mixer can be a part of the complete hash function, often the key part, but it is not the entire thing.