I wrote "countries", not "continents". I also have not claimed that the US is the most violent country.
By the metric you've presented as a yard stick for violence (Peace Index), there are only 33 countries which are worse off than the US.
Sure, those 33 countries mean the US has a worse peace index than ~80% of countries, not 100%. I concur that (based on this metric alone) the US is not the #1 most violent country.
Having said that: being worse than 80% of countries on any metric, in my opinion, is easily enough to be able to say "worse than the rest of the world" and have a reasonable expectation for that to be understood as hyperbole.