Well, the assumption is that news archives would be exist for Sentai teams, we the viewers just wouldn't see them (It's kind of hard to not report on a mecha pile driving a dinosaur monster through a skyscraper :lol), so videos and newspapers talking of previous teams from before he was born would likely exist.

But unless Miki wrote an editorial on Rio and Mele's sacrifices, where would he learn of their heroic acts at all?

Maybe he camps out at the abandoned quarry waiting for a Super Sentai related development?