Maximum efficiency in regards to what metric?
(Before you say 'money/the economy', inequality seems to be bad in that respect [1], and that's about the only thing in regards to which US system seems to be highly efficiency ;)
The US is fairly good at creating high GDP per capita. They are 8th behind: Luxembourg, Singapore, Ireland, Norway, Qatar, UAE and Switzerland.
Of those nations: Norway, Qatar and UAE are highly dependent on oil profits and Luxembourg is an outlier as an unusually small affluent nation. Ireland is also known to have an inflated GDP due to it's tax hub status resulting in corporations reporting high revenue numbers there even though that revenue is often disconnected from the country.
I think most nations would be thrilled to have a GDP per capita of $76,399 USD. When the proceeds are that much higher it takes an awful lot of inequality to make the outcomes worse. One of the biggest issues in the US is that health care is not covered by the government so it costs nearly double compared to most comparable countries and access is subject to all the inequality constraints that universal government programs mostly erase.
For example in Canada which has a GDP per capita of $51,987, the split between wages and capital is approximately 90-10 meaning that the average wage (including non-working population) was $46,788. For the US to have the same type of average wage of only $46,788 the share going to capital would have to be 38.8%.
I believe that is what they are referring to, 'the system' being highly efficient at allocating process improvements/efficiency improvements to the owners of capital leading to the income inequality you note
That's the key issue in general, not just for US. When the system is designed to extract maximum efficiency, there is little room for other stuff