I don't really agree with the person you responded to, but there is a lot of agriculture that is dependent on mined input, having long depleted the soil of whatever nutrients were there when we started. There are many places that are abandoned and barren because we did agriculture there. Land is still being dramatically degraded by many types of agricultural practices.

Meanwhile there are forests that were forest (and other ecosystems as well) 10s of thousands of years ago and just as lush and fertile (if not moreso) than they were to begin with.

Sustainable generally means we can maintain the fertility of the land indefinitely without mined inputs. Whether or not it's a completely achievable goal, it makes good sense to strive for it... Mines will become spent, theres only so much land to degrade, etc.

Often when people talk about sustainable agriculture they are talking about systems that mimic biological systems that keep those forests lush for millennia. These types of agricultural systems frequently look similar to agricultural systems of the past with more human labor involved, but less damaging waste products and fewer externalities. (Reducing pollution of the water downstream and minimizing ecosystem impact).