Again, it's petty, but I'm English and it's a pet peeve of mine.
Soccer is an English term invented in the 19th century. It spread to a number of Anglosphere countries in a similar manner to all sorts of words from 19th century England. It is currently used in Canada, the US, Ireland[1], South Africa, Australia, New Zealand etc.
The idea that soccer is an "Americanism" simultaneously requires you to believe that (a) that only Americans use the term and (b) that Americans are ignorant of the rest of the world.
The reality, of course, is that these countries are using a term that has existed for over a century and is a borrowed word from England. Each of these countries have a prominent sport containing the word football(various rugby codes, american football, gaelic sports), and therefore it was convenient to retain the term soccer.
This, of course, does not jive very well with lazy anti-Americanism however, which generally seems to be the point of the exercise.
[1] I know, I know.
The core point is that while there are regional differences none of those countries have anywhere near the same level of media influence as America, so yes, it is an Americanism like it or not. And when writing an article literally about Football for an international audience published on the internet then just call it by its name not the regional one used locally. It seems lazy not to.
Anyway it's just a pet peeve of mine (and plenty of others), it doesn't have to be logical.
The vast majority of takes on the Internet about the word soccer are lazy anti-Americanism. I accept that it's just a pet peeve for you, but I was addressing the general discourse on the term.
With regards to 538 using the term soccer, it is explicitly an American website. 538 is the number of electors in the US electoral college. The majority of readers are American, therefore it seems reasonable to use the term soccer.
You could argue that they should just use the term football and have the reader infer from context which sport it is, but then we wouldn't be having this discussion if you thought that was an acceptable solution :P
As a Melburnian I take a passing interest every 4 years in the 'Socceroos' but we have our own code that dates back to the 1850s, with influences from the indigenous game of "Marngrook". Footy is footy, soccer is soccer and we find labels such as the 'world game' or 'beautiful game' rather quaint.
Be aware that your language prescriptivism can be seen as a form of cultural imperialism.
Your country is not the world or the universe, even if everyday language indicates that. Language use seems to affect subconscious behavior, even if not intended.
Anyway, my point is, you have been manipulated through your language.
Someone in /r/BritishProblems called a zebra crossing a "crosswalk" the other day and as an en-gb native I couldn't help but be scandalised! ;o)