https://www.sec.gov/news/press-release/2020-321
"The order finds that Robinhood provided inferior trade prices that in aggregate deprived customers of $34.1 million even after taking into account the savings from not paying a commission."
What you are referring to is the fact that Robinhood can't offer a worse price than the NBBO, but the NBBO is not the best available price, there are also dark pools and brokers can match orders against their own clients.
Robinhood, like all brokers, has a fiduciary duty to its customers and as such is required to do what it can to always offer the best price to its clients instead of simply offering the NBBO, whose price serves as a worst case scenario when all other options have been exhausted. Robinhood not only failed to do that, they failed to do it while claiming in marketing materials that their execution is better than their competitors (it's not).