Right, not soldered, but no third-party is making replacement SSDs that work in the current gen.
Because it's not soldered, though, you can try to find somebody who is selling a genuine Apple PCIe SSD of the capacity that you want, and install that. Search eBay. I'm sure it won't be cheap. But if you have a 256GB SSD and you want the 1TB one instead, you can certainly try to find one of the Apple ones for sale, or cannibalize the part out of a dead rMBP, or whatever. (Apple will not sell them direct.)
-- Nathan