A lot of software doesn't have a GUI, even on macOS?
I'd actually like to know what non-Xcode command line apps you (or anyone else reading this!) is using that explicitly target macOS and nothing else - I've used Macs for only about five years and primarily for dev work, so maybe there's a treasure trove of cool stuff I'm missing.
There's osascript for executing Applescript code, plutil for working with plists, and diskutil for making DMGs. Those are just off the top of my head, there's probably others too.
I'd consider diskutil and plutil vaguely part of the "Xcode toolchain" (as in, using them (especially from outside macOS) is largely geared towards app development). That's certainly a huge use case for Darling - I actually think it would be interesting if they explicitly (but maybe not very loudly?) aimed for giving people access to iOS/macOS development without Mac hardware, rather than trying to be a general-purpose emulation layer.
I mention all this because even with that nearly 18 years of experience, I am really straining to think of a single non-GUI application for macOS that wasn't a Unix command line port. There are some Apple-provided system utility commands, and, fine, but probably of pretty marginal use in this context.
I think it's a great project, don't get me wrong, and I wish I had the system-programming chops to help. But it still seems to mostly be a proof of concept. I hope it keeps growing!