But I would argue that the rest of the world also uses primarily English online. And just by virtue of being the rest of the world, outnumbers the Americans.

In other words, of all Internet users that use English, the vast majority is likely not American.

Of course I don’t have data to back this up, except anecdotally.

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.

This list puts US at ~297m English speakers which is the largest group from one single country, that is true. But 297m / 1,537m = The US has 19.35% of English speakers globally.

You are likely also greatly underestimating current internet connectivity, older smartphones have changed things for poorer countries a lot over the past decade. For example, India has only 62.6% of people as internet users - but that's still 880m people and probably most of their 125m English speakers. Nigeria has 63.8% internet users, but that's 136m internet users. And they also have 125m English speakers, who again, are more likely to be the people who can afford an English education, and also a smartphone. And then there's Pakistan with another 100m English speakers and 70.8% internet users, etc.

Just 3 countries, (2 of which were 1 country 80 years ago) and you're close to that 300 million count already.

The list also gives US as 92.4% internet users, for what it's worth. A little less than 97% and not even in the top 20 countries by percentage, which is surprising.

The internet is less American than ever. It's just that most non-American people probably have non-English language spaces they can choose to gather in addition to the English-dominated spaces. Americans, on the other hand, are more likely to be monolingual English speakers and so they concentrate in the English-dominated spaces.

And non-Americans are all so used to people assuming American defaultism in English-dominated internet spaces because it was historically hugely expensive to get online and was overwhelmingly American English-speaking, that it's not even worth correcting when it happens the millionth time.

I've also put non-metric and US currency conversions in posts online many times. Not because I'm American or use them in daily life. It was just less annoying to convert them when writing rather than hear the inevitable multiple complaints about not understanding things in meters and dessicated jokes like "that's probably $2 in real money".

You're either overestimating the accuracy of your assumptions about your online interactions and/or seeing selection bias from your immersion in otherwise culturally isolated spaces.