I think that is likely the case. I had a similar experience a few years ago when I needed power steering hoses for my Jeep. NLA from Chrysler but several aftermarket sites listed them ... but they would all say "not available" when you tried to order. I ended up having the leaking part rebuilt by a local hydraulic shop for $50 or maybe less (the factory line set was almost $400). That was when the Jeep was over 10 years old, though.Sounds like Rock Auto does a lot of drop shipping. Their inventory says they have a part and when you order they just tell some other place to ship it directly to you. Rock Auto inventory is wrong because they didn't know GM doesn't have it until you tried to place the order.
I had the same problem years ago when trying to order a new ignition switch for a 1985 BMW motorcycle. I waited three weeks for the order to ship and finally called them only to find they looked through their computer system and turns out they didn't have the switch and couldn't ship it. It then cost 3x as much to get it from BMW, shipped from somewhere in Europe.