The U.S. didn’t dispute the horror.
It disputed the liability.

US explanation of vote (key excerpts):

“The United States does not recognize a legal right to reparations for historical wrongs…”

“We do not accept… that [these acts] constitute violations of jus cogens…”

“The United States also strongly objects… to rank crimes against humanity in any type of hierarchy.”

“For these reasons, the United States will vote ‘no’ on this resolution.”