The feedback loop will be longer than with a natural pancreas, so the control won't be as tight, but I'm not so sure it will be that loose. In any case, manual management by injecting a bolus of longer but slower acting insulin analogs isn't going to be as good as a natural pancreas either. The pump will be doing continuous injection of faster and shorter acting insulin.
If that proves inadequate, they could always add a couple demand buttons, one to indicate the patient has started eating (the natural pancreas does get signals for that) and another to indicate exercise is beginning. Still not perfect, but it has real potential to provide much better control than manual injection.