Other mistake: The trip back on foot from the nodes for Johns was way too fast, and he likely would not have had enough ammo to handle the entire trip back. From Riddick's perspective, John's left, he cauterized his wounds with some explosive debris, fought several creatures and then ascended the hill continuing to fight creatures. By the time he hit the top of the hill, Johns had made the entire trip on foot, which was long by bike, had Dahl join him, locked in the node, and piloted the ship back to Riddick.
Other mistake: Matthew Nable plays "Boss" Johns, the father of William J. Johns in Pitch Black, who was played by Cole Hauser. But the two actors are only a couple of years apart in age - Nable doesn't look anywhere near old enough to be Hauser's father. Would have made more sense for him to be an older brother.
Answer: The walls of the settlement are likely to be just as strong as the hulls of the spacecraft. It's likely they are even stronger because the settlement doesn't have to worry about weight limits. Remember also that they had to worry about Riddick hiding and getting the units back.