Several times when I've accessed a *A lounge (usually RCC) in the US for a domestic (within the US) flight I've had to show that it's part of an international (Canadian) itinerary. The rule quoted above mentions that requirement too, but there's another thread ongoing right now that also says we can access *A lounges on domestic US itineraries.

I've always assumed that we can't access *A lounges when flying purely US domestic flights.