The confusion comes from the pickups. Just like the Firebird guitar, Gibson equips today's Thunderbird bass with high-wind ceramic magnet pickups, aka hard hitters. Those are pretty thumpy and do the heavy metal thing. I think you generally want to combine them with bright strings.
More vintage style thunderbird pickups show a warm, even sound like you'd expect from a neck-through with mahogany. There is still a lot of growl with vintage style pickups, partially from the mahogany, partially from humbuckers, partially from mixing multiple pickups. Here you use warmer strings to give it some more "mass" than what comes from the thin body.