It was a joke response to a joke post about measuring things in different ways which happens around the world. It's pretty funny that you're cool with generalizing all Americans but the second someone points out that some Europeans measure differently too you're suddenly defensive about it.
I assume from the assumptions and generalizations that you're talking as a d-bag? I can see how a douche would generalize based on the stereotypes they have for nice, reasonable people, as it happens pretty frequently.