I'm not sure this is accurate. Can you share what kind of agent told you this? I heard from an agent at this time US customs can not see your detaxe info from the Pablo service or French detaxe. Definitely not an issue if you don't claim the detaxe.
I'm 100% sure because it happened to me. Fortunately I declared my purchases. But they asked for my receipts and they had printed out the vat forms from France and they had me match up every single purchase. I do save most of my shopping for my twice yearly European trips now that the euro is low, so the numbers involved were large but probably not out if the ordinary for many tpf members. I believe this is a fairly new development. I always need to stop to pay duty and this is the first time this happened.