1. Linux is not a niche in the target market for this project.
2. The project is written in Common Lisp with hard dependencies on SBCL-provided libraries[1], so there's reason to suspect it should work on other OSes supported by SBCL.
3. Sure, the presence of Makefile and sb-posix imply it requires a POSIX compliant OS, but Linux is not the only one that fits the bill.
4. The included Linux-only binary 'oras' is clearly a vendored artifact, not part of this project, and clearly an OCI client. A simple search shows it is indeed cross-platform[2].
Perhaps you should try what almost every Linux user has had to do when encountering software actually built for only one "niche" operating system that they want to use on their OS: look.
1. https://github.com/ocicl/ocicl/blob/170aff0/ocicl.asd#L34