I suppose what I'm having trouble believing is that if the piers are
not on undisturbed soil or on rock then how do they help any, won't
they shift and settle? Isn't the point to put them on rock or at least
undisturbed soil so that they don't shift or settle as much ?

On one side of the pad, they will hit rock quick, but down the hill, at
the other end of the pad, if they only go down 18", then there is still
6"+ of disturbed soil between the pier and the undisturbed soil surface.