This is not true. There is the US cup, which is 1/16th of a gallon, and then there's the general notion of a "cup" which is the crap that comes with a rice cooker or whatever.
e.g. US gallon != Imperial gallon.
The problem with stuff like cups, gallons, feet, etc. is that they don't travel.
They might kind of work in the US (and long as you're not using an appliance) but they don't work consistently anywhere else.
Which is why they're not used anywhere else. Problem solved.
We live in the age of the World Wide Web and information in ambiguous units is shared everywhere simultaneously all the time.
For example gallon are still used informally in other countries than the US.
People, particularly older people, still talk about "miles per gallon" in the UK. That's Imperial gallons not US gallons. They base their feel for that volume on that.
Same with US vs Imperial pints.
Now product information on UK Amazon isn't always rewritten specifically for the UK. Often they just reuse the blurb from US Amazon without correcting the spelling or grammar to save money.
I own an old Jetboil stove I bought well before Brexit. The inside is marked in "cups".
Any guesses if those are Imperial cups, US cups or Metric cups. I mean I bought it in the UK so they should be Metric cups but Jetboil is a US company so...who knows.
This might not be a problem for you personally but it's definitely still a problem.
Cups persist, but without much standardisation - you generally have to measure if you care to know which 'cup' size you bought. It won't often matter that much, but as I said it's intellectually annoying isn't it?
Being British I don't encounter them much, just American recipes, but I generally just mentally convert as ¼l (i.e. as though a metric cup, if anyone used that) if I'm following one.