What your friend is saying doesn't make a lot of since regarding the thickness of the clearoat. How would a few more microns protect against scratches? If your car is gonna get scratched, it's gonna get scratched.
Is he going to sand down your old clearcoat that has swirls in it? Otherwise it would show through the new one.
Honestly, I've never even heard of a clearcoat being reapplied.
If you get it detailed, hardly any clearcoat is removed. Just enough to get to the bottom of your swirls and light scratches.