Yes, this is business as usual. GM/ACDelco have suppliers for pretty much every "OEM replacement" part they sell, they just put them in a different box with a GM badge on it. The parts they select are usually (but not always) on the higher end of what's available in terms of quality, but you can save yourself a lot of money buying straight from the source. The trick is knowing who makes what's in the GM/ACDelco box...
What you've experienced backs up what I've been told by others here regarding Sunsong being a supplier for GM lines. I used them for power steering lines recently on my red '97 at the suggestion of other members. I know Raybestos is a GM supplier for brake parts like rotors/drums as another example.
If you're replacing the soft brake lines anyway, I would consider using a set of braided stainless lines from Russell or Classic Tube. They'll be more durable and resistant to expansion, which could potentially firm up your pedal a little.