Surely someone can't keep forgetting that their address on their account is incorrect. You wouldn't keep paying and ordering food to an incorrect address by accident.
But for the fact in this case that they seemed quite able to state the required code, it could be very plausible indeed for somebody with dementia or another cognitive disability/impairment to keep doing this again and again and then forget every time.
I used to have a sweet old man calling me on a regular basis - he wanted somebody else that he knew with the same name as me. He was always so very apologetic and embarrassed when I kindly and gently told him he needed the other person, but he still kept calling. In the end, when I saw it was his number, I just ignored his calls, as I knew that would perturb him less than getting through to the wrong person and feeling foolish and embarrassed.
I can well imagine somebody like him ordering something again and again and forgetting that he had ordered, forgetting that he needed to collect it from the doorstep or just thinking that it wasn't working and he had to keep trying. Maybe he might have previously lived in the house whose postcode he's quoting for orders, or forget and mix up his own postcode and accidentally transpose a couple of letters or numbers.
Plenty of older folk without any cognitive impairment still struggle to get to grips with online purchases and transactions and severely lack confidence that they've done it properly or that it's worked. It's a huge concern nowadays, how many things are online only; it suits the majority of us much better that way (including a great many elderly people who do get it), but there are still a lot of vulnerable people (not only elderly) who are just left behind and forgotten about.