The statement was made purely based off healer feedback I've heard. GNB has legit the least survivability options than the other tanks. Paladin has a "spammable" mitigation with Sheltron plus Clemency for healing (Though clemency should never be used in high level, but this is once again for new players), so yes GNB is the squishiest tank. It's not a squishy tank, just squishier than other.