Cement vs. mortar for embedding stones in drive way
I think this is more of an opinion question because the only real difference is the size of the aggregate in the concrete.
Cement / Portland cement  is the basic component in most mortar mixes and sand gravel and cement is what concrete is made of.  In either case 2” is quite thin and won’t have much strength but if the base is solid after scraping either will hold the flat stones in place.
To make absolutely clear you would never use straight Portland cement or less known types of cement as they have no shear strength without the sand for mortar, and sand and aggregate for concrete so mortar would be the only choice based on the below comments from the OP.
I have spread cement on freshly packed rock driveways but these were fresh and had no dirt yet. With rock on the ground not fresh this method would be a waste because the dirt prevents the concrete from bonding to the rock.