Delaware was treated as a colony separate from Pennsylvania already by the First Continental Congress (1774) and the Second Continental Congress (which began meeting May 10, 1775) so the 1776 date doesn’t mean much. Delaware’s ratification of the Constitution also wouldn’t have meant anything if it had not been followed by the ratifications by the other states.
The area was a Swedish colony, then Dutch, then British. IIRC, Pennsylvania coveted all or parts of Delaware, but never got it.
Something else that Plugs got wrong. I truly believe he is becoming senile. Not that there's anything wrong with getting old - we all do, or hope we do.