An example of his idea of an "American" accent was provided for us by Tittynope earlier in the thread, where Choob played "Annoying Activist Troon Student #1" (click the username to jump back to the post):
Anyway, there was some discussion about whether British actors can do American accents or not. Most cannot, but they don't get hired to play Americans in American films (unless they have some previous reputation to build on, e.g. Daniel Radcliffe), so if you only view American films you'll get the opposite impression. For example, Hugh Laurie had to work a lot to get his American accent up to standard, here's an early attempt from British television to compare to his work as House: