I'm guessing the app is showing the Priority Pass requirements over and above Plaza Premium requirements, so those Plaza Premium lounges that are under PP are showing up as requiring PP enrollment. If you look closer at the HK lounge selection in the app, there are 2, the East Hall one is the one that is under PP, which the other is not (you can check PP site), and only the East Hall one shows as "Enrollment Required".
plenty of anecdotal experience that Plaza Premium lounges only require the Amex card for entry, plus Amex TnC is clear on this as well. naturally applying for PP doesn't hurt one bit, carry both cards to maximise your lounge opportunity.