Courts also generally consider things like long-standing precedent and decades of industry practice, as well as estoppel. It's a practice that everyone in an industry has done for decades, which matches the general consensus understanding throughout the FOSS developer community, in addition to being the interpretation of the authors of the license.

As I understand it, the rationale for that FAQ entry was that a blanket statement that communication over a socket never creates a derivative work would allow for things like "what if we built a stub that just moves a shared library out-of-process and copies memory back and forth over a socket", or other similar tricks.

Of course, if Oracle v. Google holds as case law (which we'll find out any month now), then the software world turns upside-down and inside-out, and much of the industry will have cases against each other over "API copyrights"; in that world, I have no idea where this would fall.