I heard something like that once on some TV documentary, but I'm second guessing that there must have been a caveat for just the Americas, or otherwise qualified it as, "in modern history", or as a percentage of population or something.
I know that the newly industrialized machinery was a key factor (e.g. lever-action repeating rifle, developed by the Union late in the war.)
What are other known civil wars across history that had more casualties?
After many hours of Civil War documentary review trying to find the source (much of the Ken Burns series and others), I found it. Death and the Civil War (2012) (PBS: “The American Experience”: 9/12/2012).
“Never before and never since, have so many Americans died in any war, by any measure or reckoning.”
That's not inline with the original question header – but I tried to acknowledge it to be a false memory – but having now just watched the show again, you'll have to forgive me. The entire show is hyper-maudlin, and about the ugly details of mass deaths, how it was realized and processed, and the theretofore unimaginable scale of it.
If only all wars were as well-served in documentary format; vs. various forms of thinly-veiled heroic glorification.