I ask this not as a gotcha, it’s a serious question.

There is no way to write an app that doesn’t use a single piece of Apple code.

Create a new project in Xcode, simply write print(“Hello, world!) right click on the print method, click “Jump to Definition” and you get to read Apple’s code.

To say nothing of all the boilerplate stuff (and more importantly all the stuff underneath it that is automatically created when you create a new project and that is necessary to run the app.

The open source SDK you talk about might be completely free of Apple’s code, but it most definitely makes use of Apple’s code.

It "makes use of Apple's code" in the sense that it calls iOS APIs. But the resulting binary wouldn't have Apple's IP, which is what matters for the purposes of copyright and patent law. When saying something "uses" an IP, we generally mean the exclusive rights granted to the holder by law. Merely interacting with an already-existing API on an already-existing copy of iOS is not "use".

If the opposite were true, all third-party aftermarket accessories for physical (patented) products that merely plugged in to the mechanisms and didn't implement them themselves would be illegal.