That language is consistent with TDA 13-A and TDA 17.
Basically ALL hole cards must be shown, whether used in the final hand of 5 cards or not...
For example, you just can't turn 2 cards face-up in Omaha, even though you're just using 2 hole cards... you have to turn them all up, to prove you had a valid hand (no jokers, duplicate cards, etc.)...
... and also to have your hand read in the first place, it's cards speak at showdown.
This all said, I could swear that if you go back to the LV Hilton Book of Poker Rules, you only had to show winning cards, you could even showdown in holdem with one card if there was no reason to believe you didn't have two cards to start... from memory though.