Carrier fighter cover and the Force Z comparison

Why would the aircraft that destroyed Force Z not destroy the proposed larger British battle fleet?

The proposed fleet has about four carriers, whose fighters are judged capable of defeating the long-range, unescorted bombers involved in the Force Z attack.

The comparison turns on fighter cover. The bombers that attacked Force Z are described as operating at extreme range without fighter escort and without defensive guns, with fuel running low after their search. On that account, defending fighters would have placed the bombers in a disastrous position. The proposed force would have about four carriers, so the counterfactual rejects the assumption that it would face the same air-defence conditions as Force Z.