You are misunderstanding.
He’s applying a Z-only offset to the entire mesh, not part of the mesh.
Make a scalar var, plug it into the B/Z portion of a make3 node. Leave X/Y at 0, plug that into WPO.
Go into your material instance and you ought to see that as you move the scalar up and down, so does the entire mesh. You are not multiplying the offset math(s) by anything, no alpha’s, no masks, so the changes are pushed to the entire mesh.
Hope that helps.