He didn't, he's talking about when you click the "i" *on in the Ninja window that gives the list of instances etc. where you can get frags for any ninja. It lists "Seal Treasure" besides "event gains" for Tobirama even though it's inaccurate afaik. Guessing it's there by mistake and was meant to be added later, when his frags will be added in the Seal Treasure at a future update, like Narutlih said above.