There's a (slightly confused) idea, often repeated on Reddit, that because American accents are rhotic, and British accents used to be rhotic, the American accents are "closer" to the old British accents.

But some British accents (Scotland, the West Country, parts of the north of England) are still rhotic, and these accents do not really sound much like American accents. One "old" English accent, which existed prior to the rise of RP, is supposed to have sounded like this: https://www.youtube.com/watch?v=gPlpphT7n9s. Out of all modern forms of English, the one this most resembles is the accent of the West Country. But neither this accent nor modern West Country accents sound much like American accents. The only major thing they have in common is their rhoticity, which they also share with Scotland and Ireland.

RP itself is supposed to have originated in the East Midlands.

So a better way of stating it could be that American accents have retained rhoticity, something which was a feature of almost all regional accents in Britain in the past, but which some have now lost.