(Warning: also not a lawyer.) Despite the framing as a "one-off tool" instead of a mass-market "product", iOS itself is definitely an Apple product, and the FBI is requesting a custom version of it. Apple then has to sign it as an iOS update, so that from the perspective of the phone it's trusted as a new version of iOS.

The fact that the FBI requires Apple to vouch for it as an iOS product makes the "not-a-product" argument harder to maintain. (Also weakening the argument: the applicability of nearly the same software to replace the OS on an unlimited number of iPhones that the FBI may introduce later, or if leaked, a large number of iPhones in the wild.)