That does not seem like a very smart reason for why the United States and Europe cannot be compared. One is a country and one is a continent. Well sure, but one is a country of closely associated states the size of a continent with representative democracy resulting in wide political diversity, and one is a continent of closely associated countries with representative democracy resulting in wide political diversity.
Why has no one ever thought that the endless USA-Europe comparisons are invalid because one is a country and one is a continent before? Guns? Nope, cant compare. Healthcare? Nope, cant compare. Standard of living? Nope, can't compare. Economy? Nope, can't compare. No comparisons, one is a country, one is a continent. All those summer comparisons of how great smart Europe and how bad dumb USA were doing are also tossed out I guess.