Given that the books were written at least 40 years after the described events, how do we know they aren't made-up characters (or pastiches of real individuals)

Given that the books were written at least 40 years after the described events, how do we know they aren't made-up characters (or pastiches of real individuals)? Perhaps 'Mary' was a stand-in name for "Woman nobody thought to write down the name of," like "Karen" is a stand-in name today.