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?
Edit:
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.