If the engineer that wrote the plug in never used, touched, or read the source code of Discourse, then any code he writes that interacts with discourse does NOT need to be released under the GPL.
I don’t see why you believe it needs to be GPL licensed when the copyright of discourse never comes into play, unless someone, at some point later, joins the plugin to discourse.
If the Thesis case is a problem, it’s likely because they did not use clean room design, and thus their code does, in some respect, derive from the GPL code, and therefore is subject to the GPL derivative clause.
If the code doesn’t derive from a GPL project, then the code is not forced to be GPL. The key issue is “derive” and a clean room design can ensure that, legally, the code did not derive from the GPL project.
Again, this is only a theory and not legal advice, but there’s a lot of history regarding copyright, software, and clean room design that suggests this is a possible defense. IT would require a LOT of work, so I don’t know that someone would go to the trouble, but you haven’t convinced me that the GPL somehow can force a copyright backwards through a clean room design process and claim derivative work on something designed using that process.