That only comes online at level 20. And pure Druids lvl 20 are unbeatable as tanks.
Oh, yeah, nothing beats god mode druids, but the OP asked for high def without magic(I include moon druid features in the magic epigraph).
But you do not need level 20 to surpass the 21 AC, you can use shields and bracers of defense with unarmored defense as a barbarian. It only improves at lvl 20.
PD: With a plain shield (+2AC) and bracers of defense (+2AC) a barbarian with 20 dex and 20 con has 24AC. And since unarmored defense comes at lvl 1 you only need one level of barbarian to get it if you multiclass. All of that even without feats or class features, improved equipment or magic buffs that you can add to it, plus your damage resistance while raging.
There are also more builds with extra AC and more builds that do not rely on pure ac but on class features or magic. That is only an example.
