That doesn’t actually say that it isn’t the closest to a classical British accent. It only says it’s diverged from the modern one. Yeah, it isn’t the same as the classical British accent, but I believe it preserves more of the characteristics than other English accents have. They’ve all diverged, but some less than others.

IIRC, there’s an island that’s very isolated in the US who’s accent is as close as possible to a classic British accent, but it’s a population of maybe a few dozen people, if that even at this point.