There's a number of apps that run on modern Macs, that were written in C, but it is unusual to see ones that leverage the GUI.
That said, it's possible to walk from Boston to Portland (OR), but I'd rather take a plane.
When I first started programming Cocoa, I was always kind of miffed that the happy-path was: "Use Objective-C and main() should just hand-off the reigns to NSApplicationMain()". Kind of like the happy-path on Windows is "WinMain plus a bunch of boilerplate crud".
It's always felt somehow vaguely "correct" to have main() be your actual application entry point and have it manage whatever main loop you need yourself. I see that this project doesn't actually quite get there either. It's the same kind of voodoo: boilerplate code, and then cmacs_void_msgSend(NSApp, sel_getUid("run")); hands over the reigns.
I’m not sure you want to spend your time rewriting what NSApp is doing!