I may be wrong since IMNAL, but something is strange about this: 1) inspeqtor is written in Go and shares no code with Monit anyway. 2) The takedown claims violation of license. Licenses only apply to code, so patents and similar are out of this game. 1+2 don't stay together. How is a code license violated if the code is totally different between the two projects? I believe this is something that can be easily fixed…
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.