"The SFLC analysis states that "the PHP elements, taken together, are clearly derivative of WordPress code," citing the fact that they are loaded into the WordPress PHP application with include(), combined in memory with the rest of the WordPress code, and executed by PHP as part of a single executable program."
This is some kind of joke, or rather the Software Freedom Law Center appears to be engaging in a combination of stupidity, opportunism, and wishful thinking. A derivative work becomes derivative by incorporating protected elements of another work. Whatever the end user does with a plugin has nothing to do with whether the plugin _itself_ is a derivative work. Not only that US law 17 USC 117(a) gives explicit permission for end users to create "adaptations" of copyrighted works as an essential step in running computer software "on a machine".
Those protected elements exclude technical interfaces of any kind, under the *scenes a faire* doctrine and the merger doctrine. The general purpose of an "include" file is to serve as reference material so that a compiler, etc. can be compatible with provided technical interfaces. No copyright violation, even if such things as structure names and elements are copied verbatim. See Bay State v. Bentley Systems (1996) and Gates Rubber v. Bando Chemical (1993).
So it would seem that the idea that a plugin that hasn't copied any technical interface unrelated code is a "derivative work" is entirely specious, fictional, and without any sort of legal foundation whatsoever. The SFLC website certainly contains no trace of an argument to the contrary. The common belief on this point is a combination of rumor and innuendo that such entities as the SFLC and the FSF propagate to an unwitting public, on no apparent basis other than pure opportunism.