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.
I think they're drawing some fairly speculative conclusions here. There is good evidence that the social system differed from other contemporary civilizations, but its still quite speculative to draw the conclusion that it was truly egalitarian or lacking in powerful elites. They may have simply exercised their authority and power in such a way that is not obvious to us from the archeological record.
More importantly, the idea that such a system, if it did exist in the way that we imagine, contributed to the city's success, is entirely without basis. Considering the lack of similar examples in the archeological record, it seems more likely that this particular city would have succeeded in spite of its more egalitarian social hierarchy, rather than because of it. It's not exactly an uplifting interpretation, but academics don't get paid to be uplifting, they get paid to be accurate.
All of that being said, my conclusions are also speculative, so it doesn't entirely rule out the possibility that they were egalitarian and successful because of it. If we imagine that was the case, the next question becomes; was that a sustainable and stable arrangement, or was it a fragile system such that external actors or changing environmental conditions could easily disrupt and ultimately destroy it?