Wrong is perhaps a bit harshly expressed - it is better to let the stones protrude over the base slab in order to insulate it flush with the wall. If you now stick it on the outside, the terrain must then be 10...20 cm above the top edge of the base slab (raw floor) in order to still be able to cover the insulation. "Due to rain" I would always set the base slab slightly above the terrain.