I think I heard it on a podcast. Essentially what they were saying is that George Washington was not the "first" President. John Hanson should have that honor. How "true" is this and on what basis could you consider the US just a successor nation of the Thirteen Colonies and not a completely different entity? I mean on paper the US as a country is older than say Italy but no one denies Italy as a country is older than the US.
Technically speaking, although in some sense the Constitution was regarded as a use of the amendment process of the Articles of Confederation, because it only required nine states to ratify itself to go into effect rather than the unanimous vote required by the Articles, it can't legally have been an amendment to them. (Well, until the remaining four states ratified it, at least.)
However, the fact that nine states (and eventually all thirteen) did ratify the Constitution, and that it wouldn't have gone into effect otherwise, shows pretty conclusively that "coup" is a pretty inaccurate term for the process of replacing the Articles with the Constitution.