First, the APIs don't expose raw controls for everything, but it exposes a heck a lot more than the stock camera app.
Second, for high-value developers like Blackmagic, Adobe, etc, there are very regular communication between engineers and product managers in both respective companies. I wouldn't expect private APIs (although they have certainly happened in 3P apps over the years; just check undocumented entitlements of IPAs from major developers), but these apps can be years / iOS releases in a making with a lot of Apple support.
Things like the parameters for the optical image stabilization algorithm aren't settable.
I'm therefore surprised a 3rd party can make a camera app that 'beats' the built in one.