No, its right - the scalpings will be vibrated so that they're hard packed, making a relatively solid surface. The membrane is to prevent anything from trying to grow up through from the soil beneath. You might just want to ask precisely what they mean by 'weed membrane' though - under that weight of stones, they should be using the much thicker, usually white, material used under driveways rather than the thin black stuff.

Over time though, you will find some weeds germinate in the top layer of gravel/stones, as it collects soil particles in the wind.

Do be aware that that pebbles/chippings as a lawn replacement are not maintenance free. In time weeds will grow and look a mess after a couple of years unless its raked over fairly regularly