In general humans don't have perfect recall. Even people with what we might call a photographic memory don't have the ability to memorise millions of lines of code and output them with little effort.

It hinges somewhat on the concept of how much you believe things are being learned and how much is just pattern matching and borrowing a solution from memory. Certainly in the early days of Copilot it was possible to get it to output chunks of open source code near verbatim.

I think, generally, people are probably closer to believing that there is some kind of reasoning being carried out by these models than in those early days but it would also be easy to strip all of the immediately identifiable comments etc from the training materials to make it harder to detect.