Misleading title, author equates "English" to "British English".
To assuage American readers? That's a rather condescending attitude, don't you think?
The very word "American" is presumptuous. Imagine if the Chinese referred to themselves as "Asians" and their country as "Asia".
I've taken to using estadounidense until something better comes along in English. ..or Spanish becomes the main language in the US :-) Australian English has/had the rather pungent seppo - rhyming slang for septic tank = Yank. But estadounidense is at least accurate.
And should the non-clarified form of the language not automatically be assumed to be the original form of it? Compare Spanish Spanish vs Mexican Spanish, it feels an unnecessary clarification
Same for Spanish, when I'm talking about that language it doesn't even cross my mind that someone might think Mexican Spanish.
But English is not my first language so it may be different e.g. for Americans.
Supposably, Americans could care less that other English-speaking countries are closer to en-GB than en-US, despite the relentless onslaught of inconsiderate software defaults.
So they do care?
The meaning overlaps a lot with supposedly, which is much more-commonly used, so it sounds a bit odd.
It is a synonym for "conceivable", whereas supposedly means: "according to what is accepted or believed to be true"
It is especially reasonable when used this way by an English writer writing for a British publication.