That description applies to both the US and UK, perhaps others, and it's hard to be unaware when you're connected to a global computer network. Just google "site:uk mpg".
"Put simply, all MPG does is tell you approximately how far your vehicle will travel based upon a single UK gallon, which equates to approximately 4.55 litres of fuel"
This particular thing, I'm aware of because there is perennial confusion over comparing mpg given that Imperial gallons are larger than American ones. If people just used L/100km....
Chauvinists go to ridiculous lengths to magnify small differences.