You'll notice I didn't mention copyright, and that's because I don't think it's relevant. I think your analysis is correct. Keep in mind too that Stockfish – the engine I work on, although I wasn't at the time – was the subject of a high-profile defense of the GPL in Germany: https://stockfishchess.org/blog/2021/our-lawsuit-against-che.... That case was different as it involved wholesale copying of the source code, rather than porting ideas. Ideas shouldn't be copyrightable.

But copyright and plagiarism are orthogonal, and questions of morality are much more tied to the latter. It would be illegal (in the United States) for me to publish a copy of Nineteen Eighty-Four, but not immoral. It was not immoral, in my view, for Aaron Swartz to try to liberate JSTor articles.

That doesn't make plagiarism acceptable, either. And therefore, I'm willing to call it out when I see it.