I believe he's suggesting that caches should only be allowed to read from persistent storage and that values read from cache should not be assumed to be current

I believe he's suggesting that caches should only be allowed to read from persistent storage and that values read from cache should not be assumed to be current. For example, if I read $FirstName, $LastName from some cache lookup for a user and then go to update $Address I should not write $FirstName and $LastName along with it as a "whole user update" sort of thing; just update the value that you need to.