But they could have still done all that and still had some advancements in the culture. Sort of like Chakotay's people in Voyager. They respected their traditions, still wore the traditional clothing. But they had and were aware of technology.
Realistically, yes. But it was an episode of a '60s TV show, made for '60s audiences who had preconceptions about what "Indians" were like -- preconceptions shared by the writers and producers. A lot of TOS aliens, especially in season 2, were just stock '60s TV ethnic tropes transposed into outer space, and this was one of the most literal instances of that. When you watch other '60s shows, you see how much TOS was steeped in the preconceptions and tropes of its time.
Maybe they really were just doofuses after all. Perhaps that's why they aren't around anymore. They forgot their own asteroid deflector.
Who says they aren't around? I still think they're the Vians.