I can't say that this is entirely incorrect, but it's at least full of inaccuracies. RP has indeed been evolving as the accent of the English upper class, but i

I can't say that this is entirely incorrect, but it's at least full of inaccuracies. RP has indeed been evolving as the accent of the English upper class, but it originated in the accent common in the English west midlands and as far as I know has been non-rhotic for quite a long time. Rhoticism in Germanic languages comes and goes, and since Britain always had many different regional and social accents, it's entirely possible that some of them were non-rhotic still before the stipulated "divergence" began. While in England, accents that are mostly non-rhotic were the most common, America had a large community of immigrants from Ireland, where the accent is rhotic, from southern Germany, where the accent is rhotic, and from many non-English speaking countries (learners of English as a foreign language often default to rhotic pronunciation since it's closer to the way the words are written). Most importantly, there isn't such a thing as a British accent, but there is (with only minor variation) such a thing as an American accent. Americans often don't realise how diverse the UK is simply because they're not used to this sort of diversity from their own country.