What the rule actually checks in Stony Cliffs Are Cool

The replacement is not applied to dirt in general - it is targeted, which is why ordinary ground is unaffected.

A block is only changed when it is dirt that is genuinely part of an exposed steep face, with stone or similar material already present around or above it. In practice that means the vertical cuts on a hillside qualify, while a gentle grassy rise does not. The block on the surface stays grass, so slopes still look alive from above; it is the cliff face that changes.