Earlier quoted context omitted.
Yes, this is basically considered fact. American is old-English. The same with Québécois and French.
I am not sure about these "facts". Why would quebec accent stay the same for centuries? Languages are unstable, especially their pronunciation. Consider also the extraordinary diversity of accents in uk. Did only one cross the Atlantic?
Believe it or not, there is an entire field called linguistics that studies human language and how it evolves.