I see nothing wrong with that app, since it’s purposefully convoluted. Apple’s application frameworks are written in and meant to be accessed through Objective-C so trying to write an app in C is going to require a lot of hacks and additional complexity. A simple application can be written in under a dozen lines of Swift or Objective-C code. (For an idea of how insane what you linked is, consider trying to use a C++ library from C: you’d need to hand-mangle the function names, and if you want to pass in parameters that were C++ objects you’d need to lay out their vtables and fields correctly, and…you get my point. It’s not anywhere near representative of what the normal development experience would be like.)

It's not "purposefully convoluted", and I'm not talking about "normal development experience", but rather the larger amount of non-essential complexity it shows; and as anyone should be aware, complexity is always an opportunity for bugs, regardless of whether it's something you would see or write or even be aware of in the "normal development experience". Combine that with Apple's attitude of constant churn and it's not hard to see how bugs can multiply easily.

Using a C++ library from C is not hard because objects map to structures in a straightforward manner (and in fact early C++ compilers were C++ to C translators), and you would need to use structures anyway, unless the library was obtusely designed with nonessential complexity. That's how the Win32 API works too (with the exception of the bloated COM stuff.)