The problem lies in choosing where to draw the line. People often assume the acronyms they use every day are well known, and don't think it's necessary to expand them. Case in point, you don't define APA, because you (correctly) assume I know what it means.

This article doesn't look like it was written for newcomers to the field; it appears to be written for hobbyists interested in having yet another cheap SDR at their disposal. The target audience knows the acronyms, so the article doesn't bother to expand.