I don't think they mean you can't use a flight that is code shared. They mean that you must book a flight using the operating carrier's code. But yes, as has been discussed many times previously, the PP Elite companion ticket program is nowhere close to as good as it sounds on paper (not surprising since this is a $75/yr card). IME, the flight quotes I get would save me $40 or so from just buying the two tickets outright. Deceptive? Probably. Useless? Not really.

Which city pairs are you looking at?