Some History of Pepper's Tribe on Orvunos
Maddie has history books on Anuipuk about Pebbles' people who lived in the cave system on Orvunos, so there are records (from the outside looking in) as to who Pebbles' people are. It's possible that people have looked for this 'lost tribe' of Orvuns without success of finding them for years– because her tribe secluded itself intentionally, because the cave system is basically a labyrinth, or because of some kind of magic barrier, it's unclear.
Pepper's tribe has always lived in a cave system on Orvunos as far as she knows, but her ancestors used to be in contact with other Orvuns until they disagreed over whether to maintain contact with the newly discovered island (Treowa). Pebble's people probably knew that if they went to war with Treowa on their own, they'd lose, but they no longer had the support of the other Orvun tribes, so they closed themselves off... and either intentionally or unintentionally forgot about the rest of the world.
I'm not sure how long ago it was that Orvunos came into contact with Treowa, but there may be other records of her people (outside of Maddie's book shop, especially on Orvunos), though. Just Pebbles' tribe has kept no record of the outside. Again, it's unclear whether that's intentional on behalf of her tribes elders and her ancestors or if it has simply been lost.
It's highly likely that her ancestors came into contact with the shifter inhabitants on Orvunos, which is why Pepper has hand drawn children's books with old tattoos like Abel's. It's also unknown at this time what relation Pepper's human ancestors had with the attackers that Abel faced years ago.