Sounds to me like the builder is just trying to get a better understanding of the work.
If they'd been to site and had a look, it would be a question that might indicate that the builder wasn't experienced in this kind of work. But if they've not been to site, seen what you're asking them to remove and are just working off photos, i wouldn't view the question as anything to be worried about.
If you want the smooth finish as opposed to pebble dash and the pebble dash is in good condition, why not just render over the top?