This is a really cool read with lots of very strong results, but "show" doesn't seem like the right word for the specific claim the article makes from the paper. In grad school we had a professor who led the first year seminar who drilled into us the importance of using the right word to communicate inferential strength. "Is consistent with" is weaker than "suggests" is weaker than "shows" is weaker than "proves" (really only mathematicians should use "prove"). Section E3 on this website has a similar hierarchy.
My "speak up in seminar" reflex was going off here because this article jumps one - possibly two - whole levels of inferential strength from what's actually written in the paper.
In the paper, the inferential claims in the "communal effort' part are:
These differences clearly suggest a lack of evident social stratification...
further revealed no clear signs of social stratification
It's possible I missed a stronger inferential claim about the communal aspect - Please correct me if so!
I think "are consistent with" or "suggest" would more accurately communicate the strength of the results. The evidence presented that the drainage system was a communal effort is that the houses were the same size and the graves didn't seem to be differentiated. This seems like absence of evidence for a state authority/hierarchy, not evidence of absence.