He meant that, since the contents of your deck aren't public knowledge, you can choose to search for no cards and just shuffle your deck afterward. Your opponent doesn't know what's in your deck, so you can choose to not search for anything (even if it is there).
With abilities like Slowking GE's Trump Card, though, you can't do that because you are searching for "any 1 card". If there is at least one card in your deck you must take it, since it's not a specific kind of card being searched for.