That many druids should easily be able to make enough goodberries to feed all those refugees with no real strain.
If there really is only a dozen or so tieflings you'd be right, but populations are almost always dramatically scaled down in video games.
Plus Goodberry needs spell slots and we also see the druids making heavy use of Talk to Animal and Cure Wound as part of their regular druid duties. Any magic going into feeding and healing the tieflings is magic that can't be used to deal with the day to day maintenance of the grove and the surrounding wilderness and populace.
Not to mention they need those spell slots to fight too. The grove has been getting harassed by various monsters ever since the tieflings showed up. Feeding the tieflings literally makes it harder for the druids to defend their home. As anyone who's played a Moon Druid and had to sacrifice a perfectly healthy wildshape form to cast goodberry to save a party member can attest to.