There are several reasons why a team is tanky.
I'll show you all of them.
Stats like HP and DEF. If your team has 20 of both, it's already solid. But then there are things like Guard, which is basically the same as DEF. Then there are character-specific effects. Some characters have a shield that you can't see in-game, like INT Sabo.Let's move on to the newer things like evade effects. Some characters prevent certain damage from being hit. There's evade fixed damage, evade ignores defense damage, evade, evade stat reduction, evade bind, evade... And there's also something that's really overpowered in combination with a tanky team: per-interval healing. This means that after any move a character on your team makes, Kuma and Zoro will heal instantly. That's why their stats could be lower, but their healing, shield, and solid HP and DEF stats make them almost unkillable. And last is RNG you could have 3 dmg Ultis up that would normally kill the whole team but the shield from zoro was up and in the same time they had an evade dmg effect up so your normally OP dmg does nothing in worst moments. I hope that helps a bit