Perhaps; we certainly have the largest reported imprisoned population by those standards, but we also know that there are several countries whose numbers really, really can't be trusted. But, I didn't take issue with “the US police and prison industrial complex is the worst in the world” [0], I took issue with “it would be impossible to make it worse”, since it manifestly has been worse in terms of both abuses within the system and unjust motivations for getting people into the system.

[0] which is not to say I agree, even setting aside potential issues with reporting of incarceration rates, because I'm not sure you've weighted situations where the “police” (and often the prison system, military, and other institutions) are heavily in league with organized crime, such as drug cartels, and directly involved in carrying out and covering up large scale murder on their behalf properly against incarceration rates.