An interesting theory.....i suppose a close cousin of " the leans" which i have experienced several times but overcame immediately by forcing myself back onto following the AI.
The pilot assistant had many years commercial instrument flying under his belt as an ATPL holder. One wonders just how much instrument experience the captain had? Apparently his first command and a PPL examiner......rather suggests much of his " considerable experience" would have been gained at a PPL level flying around under visual flight rules.
One of the pilots might have suffered the illusion but surely not both?
Shame this old machine didnt even have a cvr. We may never know what happened....which would be the worst outcome.