The Rutterkin (from Mordenkainen's Tome of Foes, pg. 136) has the following attack:
Bite. Melee Weapon Attack: + 4 to hit, reach 5 ft., one target. Hit: 12 (3d6 + 2) piercing damage. If the target is a creature, it must succeed on a DC 13 Constitution saving throw against disease or become poisoned. At the end of each long rest, the poisoned target can repeat the saving throw, ending the effect on itself on a success. If the target is reduced to 0 hit points while poisoned in this way, it dies and instantly transforms into a living abyssal wretch. The transformation of the body can be undone only by a wish spell.
(Bold emphasis mine)
A level 3 Paladin has the Divine Health class feature (PHB, pg. 85):
By 3rd level, the divine magic flowing through you makes you immune to disease.
Recently, our party's level 3 Paladin got bitten by a Rutterkin and failed the Constitution saving throw. The DM has ruled that he is now poisoned (although hasn't told the player yet). The player (correctly) suspected that it might be something disease related, but the DM says it still counts (he has explained to me privately that it's because it gives the poisoned condition, not a disease).
Would a Paladin with Divine Health be able to automatically succeed the saving throw against a rutterkin's bite?
On the one hand, the saving throw is "against disease", but as my DM points out, the effect is gives is the poisoned condition rather than a disease specifically. Which interpretation is correct?