As the Wanderer wrote in his memoirs:
Together, our little group moved north, away from the Vault, and away from that old life. Slowly, I taught them what experience had taught me. And together we learned to thrive.
The tribal garb could (presumably) be an affectation - a symbol of the
Wanderer's rejection of his jumpsuit.
Equally, there is no "regression" - Arroyo is a simple farming/hunting community, albeit with a strong religious/superstitious bent and an introspection or self-reliance that developed from isolation. This could, equally, be seen as a rejection of the old technology and the ways of the Vault. The community is doing well until a drought causes the crops to fail and animals to become sick. None of that seemed inconsistent or implausible to me.