You are right. Regardless off who orders them the makers are often the very same jewellers such as cartier that have been used by royalty, aristocracy and the wealthy of Europe for decades and some even longer .
Even so, I think the origin has more to do with privacy than secrecy. Catherine has worn classic historical jewels from HM, all of which were readily identifiable and whose history was a matter of record. The delightful earrings worn by Meghan have no provenance 'except', I believe, that they are royal as we know that the BRF frowns on borrowing from jewellers. Because their design gives a possible style date we are left assuming they are from Camilla's collection.