What I am saying is that of all Internet users that use English, Americans are by far the largest group

No, they didn’t misunderstand. It is you who are massively misunderstanding. You are suffering from the erronous assumption that people who speak English on the internet are native English speakers when that it is not so at all. People speak English on the internet because it is the largest commonly understood language. So people from non-English speaking countries are using it as well. And there are a heck of a lot more non-native English speakers in the world than native English speakers.

So you are most likely at any time on the internet to be speaking to a non-native English speaker, and thus definitely not an American.