As Mohenjo-daro prospered, the gap between its richest and poorest households actually shrank. Its shared infrastructure, fair trade systems, and lack of powerful royal elites suggest that equality may have helped fuel the ancient city’s success.
So the absence of evidence does not necessarily indicate an evidence of absence in this case
we don't have a complete absence of evidence, we have some evidence and it leans in an egalitarian direction.
ultimately less civilized
"Civilized" is moral and political judgment, not a scientific one. there are plenty of reasons to judge our current society as "uncivilized". This hints at a desire to discount egalitarian societies rather than to construct a nuanced vision of the time.
Catalhoyuk may have been fairly egalitarian, but it would still be a far less relevant piece of evidence
We can choose relevant time periods if we like, but I'm making a simple, broad argument that there were egalitarian societies we can point to to undermine the argument that hierarchy is "natural" or "necessary" in society. I'd count Catalhoyuk for that.