Certainly sounds like malarkey to me! If it's the same model number, it's the same sink!

If the model numbers are exactly the same, it's the same sink.

Many plumbers don't like it when you source your own materials as it prevents them from taking their cut of the sale. When they provide the materials, they buy at a lower price at a plumbing supply company then put a decently-sized margin on it for their own profit. I found every item from my initial GC estimate online much cheaper than the plumbing bid.

I've had a few plumbers tell me they didn't mind me providing my own supplies, but then padded their labor charges to make up for the loss in profit so the overall costs ended up about the same. Their theory is that they warranty the products if they provide them and will handle returns of defective products, etc. I decided that part was easy enough for me to do myself and save some money. After much looking, I found a plumber who didn't charge me extra for labor.