Your profile bio mentions Arch, Thinkpad, and SDRs. Just from that, you can likely expand your skills section to spell out the operating systems that you’re familiar* with (including others like Windows or Mac OS, if you’ve used those), possibly with inclusion of anything related to coreboot (if you’ve done that), and can include radio technologies in its own section under either skills or interests.

Don’t sell yourself short, because if a job posting is upfront that they’re not looking for hard skills as a prerequisite, then the assignment is to present yourself as a candidate with depth. They’re clearly looking for candidates in a non-conventional manner, so you should showcase all your non-conventional aspects. Don’t be afraid of including “niche” details like RTL-SDR, because they can easily do a web search to figure out what that is. But do contextualize it as a tech interest, because if maybe the job requires working with complex, one of a kind tech, then they might value the candidate that self-learned.

Regarding “familiarity” on a resume, to be “familiar” with XYZ just means that you at least know what it is and have probably used it. Listing out some skills on a resume means, at least to me, that you’re familiar with all of those skills. It’s different if you’re actually an expert or fairly accomplished with something, in which case you’d describe that skill in detail, usually giving an example that you’ve worked on. I recommend including everything you can think of, even the mundane stuff like Microsoft Word; you’d be surprised how many people don’t know how to use Word or Excel.