The paper does imply that, but I would disagree that it is more hardened. I would guess that this strict process per tab model is Safari's attempt to get some degree of isolation despite not having true site isolation.

> Given that both the calling window and the newly opened window share mutual references to each other, isn't the next side channel attack lurking around the corner, even if these windows are rendered by separate processes?

Non-same-origin opener references only allow very restricted operations. It is possible that there are undiscovered issues, but it is a lot less powerful than running in the same process. It isn't like having a raw pointer from one window to another.