Design "theorem".

The (hookable) eval/Function function objects, the global object, and the realm all have to be in 1:1 correspondence.

"Proof".

(a) same realm ==> same Function

Since Function is reachable by function literals, it is tied to a realm.

(b) same realm ==> same eval

Since the semantics of the direct eval syntax checks the object identity of the original eval function object, it is tied to a realm.

(c) same realm ==> same global object

Since Function and indirect eval are implicitly "closed over" a global object (i.e., they evaluate code in the scope of a fixed global object), it shouldn't be possible for two loaders to share a realm but have different globals, as the Function constructor would use the wrong global in one of them.

(d) same global ==> same realm

It's pretty hard to imagine writing sensible code where new Array and [] or new Object and {} have distinct prototypes. While technically we could imagine breaking this invariant, real-world JS depends on basic built-in classes like Array being there and behaving predictably.