To me it's shorthand for "It took years of practice to get good and I can't explain the process I use. The problem appears too complex for a scientific approach to be worth the effort. I certainly can't think of any polynomial-time algorithm and my experience has led me to doubt that one exists."

It's a caution for beginners. The expert is basically saying that getting good at it will take a lot of work and finding a good systematic approach is likely to be a dead end.

They may not know how to deduce what the perfect painting would look like, but that doesn't mean that it's not possible.

One can certainly brute-force the issue by creating every possible painting and comparing them. Whether this can be done efficiently is another matter altogether.