Europe is not just the "European Union". I know many don't like Russia at the moment, but its western part is Europe. Same for Ukraine, also not part of the European Union, but it's part of Europe (the continent).
In the context of European "genetics" keeping Russia and Ukraine out of Europe is basically non-sense considering the Slavs kind of come from there :)
For a more accurate reference point, the EU's area is about 4.2 million km and the contiguous USA (so without Alaska etc) is about 8 million, so the EU is about half the area of the contiguous US.
Europe is 10 million km - why to skip some countries just because they are not part of a "political" union?
This article is about analysing the genetics of people living in Europe, so the entire continent must be considered and "historically" Europe extends until west Russia.
If you consider only the European Union, you kind of exclude the British, who share a lot with the rest of the continent.
To match the guy I was replying to, and because the USA is primarily a political, not geographical construct so it makes sense to compare it to another political construct in the EU.
EU is a lot bigger than Alaska and there's a lot of Europe it doesn't cover.