If you rewrite Harry Potter in a different script, language, or some plot twists, it's still "derived work".
That Inspeqtor works with Monit configuration files is definitely not proof of copyright violation. Neither an admission of "heavy influence". As far as I know, the only thing that would prove that this is a derivative work (and therefore possibly a copyright violation) would be a comparison of the source code, one demonstrating a number of features in the Inspeqtor code base that show that Inspeqtor's implementor was doing something more like a port to Go than a compatible implementation.
That's sure not part of the takedown. From the way it's worded, I worry that Monit's authors are not totally clear on the distinction between patents and copyrights, and that they have not taken the advice of a competent IP attorney before taking this step.
And if I called that story Star Wars?
Rewriting a program in a new language is a large change in its own right; even more so, if it it uses features in the new language which were unavailable in the original.
Antirez is taking a very narrow copying must include a figurative Ctrl-c Ctrl-v. Where as tomp is taking the wider and I think more legally supported(at least as far as art goes) stance that a work can be 100% your own effort but still be a derivative work if it was inspired by an existing work.(i.e. taking a photo of a sculpture is a derivative work )