The story we see only follows one small village. As far as technology the village is more advanced than many places in rural American when people were walking on the moon in 1969 from the same country. Kataan didn’t even make it to Apollo level spaceflight as their probe was uncrewed.
We’re also told that they have lots of time (multiple decades?) of knowledge the planet won’t survive. A planet can move many mountains when it knows there’s no point in building for a future. The entire planets resources can go into the project to preserve the knowledge of the culture because they know none of them will survive.