I have to think this is just another case of one hand not talking to the other with regards to an AA marketing initiative and what gets put in the press release, or a disconnect from what gets put in the press release and how situations will be handled in the real world.

For a flyer with no frequent flyer number in the reservation then sure, tell them if they join AAdvantage then there will be a better rebooking option.

But for a flyer traveling with another oneworld frequent flyer number in the reservation, elite or not, I can't imagine anyone from AA actively "punishing" them in terms of less rebooking options just because they don't have an AA number.