There are many examples of concert pianists who have known music 'by heart' but had human failure during a concert: some minor, some disastrous. A very large disaster occurred to Yundi Li during a prior concert. He is therefore using the iPad to have access to the score to avoid this type of problem again. Yes, he has it memorized, but at the same time, it is a fallback if needed. He wins my applause for this!
Do you have a link to this disastrous performance or news bit, im very intrigued.
https://slippedisc.com/2015/11/breaking-yundi-crashes-out-of-chopin-concerto/