His point is charcoal doesn't decompose which is why we find soil with 5000 year old charcoal in it. It's basically permanently sequestered.

It does sometimes in forests that burn. And generally, forests keep building up soil, it's disturbances and things like erosion (sometimes worsened by fires) that counter this buildup. But mature forests tend to do a lot better at resisting erosion and catastrophic (erosion-causing) fires.

Forests do not infinitely accumulate soil. When we dig into the ground in a mature forest, the layer with organic remnants is not particularly thick. Dead organic decomposes and returns back above the ground.