Checked my rank when I woke up before heading out and noticed the top player was using a Lancelot with a full R team with R main weapon and eidolon. I'm guessing their roster updates as they change it and they were just screwing around or something.
That's what I'd assume too. Saying it was by only using natural regen seems like an exaggeration. Their amount of PP is likely not something you can get with 72 BP from natural regen running from the start of the day, meaning running low level experts for little amounts of PP.