Now I understand where the misunderstanding started, however, I cannot agree with this approach.
I am aware that some people use term "Lack of public IP" for "public dynamic" but that is just some people being stupid. The term has only one interpretation and that is the one you said yourself - RFC1918 address on his device, (because it is behind CGNAT as he said)
As skilled person, you should never ever assume/accept this interpretation. Every time you do, you are spreading this mistake further which increase everyone's confusion.
Anyway, thanks for explanation. Have a wonderful day