Historically, this is completely untrue. The post-Reconstruction U.S. famously had all sorts of laws designed to lock up black people for being black, as well as officially tolerated (with public officials often taking part) terror killings of black people just for being black. Even after Jim Crow, the War on Drugs was was explicitly designed to disproportionately lock up black people:

"The Nixon campaign in 1968, and the Nixon White House after that, had two enemies: the antiwar left and black people," former Nixon domestic policy chief John Ehrlichman told Harper's writer Dan Baum for the April cover story published Tuesday.

"You understand what I'm saying? We knew we couldn't make it illegal to be either against the war or black, but by getting the public to associate the hippies with marijuana and blacks with heroin. And then criminalizing both heavily, we could disrupt those communities," Ehrlichman said. "We could arrest their leaders. raid their homes, break up their meetings, and vilify them night after night on the evening news. Did we know we were lying about the drugs? Of course we did."

Even if you argue that today this intent has been largely wrung out of the system (which is not a given, and does not address the remaining disproportionate effects of the War on Drugs), there's still the question of when exactly the U.S. stopped doing what you're calling genocide and started doing non-genocidal mass incarceration.