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.