Hi, you can get your contractor to remove the paint and skim coat to reveal if it's concrete slab. Just start at small portion of the wall. If he don't understand, u can show him my photo.

For removal, they have to chip away manually. Can't use chemical. Do note, cement concrete will gives off cement dust thus u need to seal it with waterproofing. I have 3 coats of waterproofing (susu).