Tom, GREAT question and one I asked myself two weeks ago. I'm not sure this will answer your question but this is what I did.

I had the same message two weeks ago and i traveled DCA-ATL-JNB and just got back today, actually. Two weeks ago I took a day trip to/from Chicago and took my passport on that trip just for the purpose of scanning it at a kiosk. The DL ticket agent looked confused when I told her about the msg on DL.com but she did scan my PP and it saved to my profile. I WAS, then, able to use my mobile BP for DCA-ATL-JNB last week but was not able to use the mobile BP on the return. So I'm not sure if she just verified my PP for just the one trip or whether I'll be able to use it in the future. Perhaps others can answer that. I also note that I have not seen that msg in my profile since then.

Looking forward to other replies and thanks for posing the question.