The problem with the French is that they don't have a word for entrepreneur. Apparently it is a false quote, but I still like it!
He says “I can tell you that the prime minister [Blair] never heard George Bush say that, and he certainly never told [his friend] Shirley Williams that President Bush did say it. If she put this in a speech, it must have been a joke.”
While Blair / Campbell have an obvious interest in denying this (it's not good to be rude about other countries leaders), we might reasonably be suspicious of Campbell's denial.
This is still useful if our interest is "When was this said?" because we know Shirley Williams talks about it, so e.g. if some future historian is wondering about a claim Trump said this to Johnson, they've got this record that well, this particular "American Presidents are idiots" quote certainly pre-dates that.
There a lot of interesting turns of phrase where that is primarily what we care about - did Franklin say that stuff about hanging together or hanging separately? Perhaps not. This play on words does however seem to originate in that period, you won't find it much earlier than the American Revolution, while by the 19th century it's commonplace.