British Accent

I learned today from John McWhorter at Lexicon Valley that British and American accents were indistinguishable during the Revolution. The American Accent Came First.  This seems impossible to me, given how different British accents are from each other, as far back as we can trace. So perhaps it means the type of British people who ruled and policed America up until 1775.  But colonists speaking to each other about the possibility of spies note often that they can't tell whether someone is British or American by listening to them. The British accent distinct from American did not start differentiating until after 1800.

Wouldn't the Scots-Irish and English Borderers in Appalachia sound different?  Oh wait, if the coastal accents of both countries were that similar, then people on the frontier would detect even less difference between them. I do recall hearing that the coastal colonial accent was consistent up and down the Eastern Seaboard.  I trust McWhorter, but I'm having a hard time getting my head around this one.