Unless CNN's contract with cable operators is vastly different from what other channels have... I don't see why there would be any issue with HBO Max making CNN programming available (on a delayed basis). I note that HBO Max already carries some programming that originally aired on CNN, such as "This Is Life with Lisa Ling" and some feature-length documentaries.
Why the idea of CNN+? Because they presumably still can't stream CNN programming live, but they thought the brand name was enough of a draw that they could make money off selling it as a separate streaming service with a certain amount of value-added content.