I love the United States' flexible decentralized model. Different states have sufficient autonomy to tinker and try different approaches, allowing the best ideas to emerge from the collective. It's one reason why the US is such a dynamic place, similar to Europe but with the additional benefit of unified language, culture (broadly), economy, and high level regulations. It's truly a great system.
It's not necessarily as flexible as to encompass such sub-federal cooperation: "No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay." https://en.wikipedia.org/wiki/Article_O…
It also sounds like [2] things are rather flexible in what approval is and when it is obtained. It's plausible that they expect to ask for approval, or rely on it being unlikely that congress explicitly disapproves, or perhaps they expect to obtain approval in the future.
[note: I've linked to a point in the opinion discussing Congressional approval]
1: https://scholar.google.com/scholar_case?case=103881997639573...
2: https://www.gsgp.org/media/1313/understanding_interstate_com...