Americans don't claim to use imperial weights and measures; they use customary weights and measures, which were also used in the UK prior to the creation of imperial units with the Weights and Measures Act 1824.
Many Americans do claim to use imperial units. They’re wrong, but they do claim it.
The (incorrect) claim is indeed made in every single metric vs "imperial" comments section I've come across.
There are many people in America who do not know the difference, the mistake is fairly common.