There are some very dubious claims in their DMCA takedown notice: > The work, Inspeqtor which is hosted at GitHub, is far from a “clean-room” implementation. This is basically a rewrite of Monit in Go, even using the same configuration language that is used in Monit, verbatim. There is no concept of "clean room" in copyright law, the only issue is whether or not substantial copying has occurred. While it may be easie…
Derivative work is not a factual statement of a work, but a narrative statement in how it came to be. It helps to look at other arts (film, books, music) in order to examine the legal theory as it is used.
Lets take a manga comic written in Japanese language, using Japanese culture in order to express a story. That is then rewritten into a movie script for an American audience, using language and culture that is familiar with that market. The two works might in the end share nothing substantial in common, but the narrative defines the movie as a derivative of the written manga.
For books, fan fiction is simply defined as derivative works. The word itself defines a narrative that do not include copying but a process which bases one work on an other.
For music, style from a couple of notes has been know as enough for derivative work to trigger. If a song provide an association of someone else song, the narrative has an implied infringement on the original work. Copying as an act is more or less completely absent from those cases.
So for all the different copyrightable arts, software seems to have the most narrow interpretation of derivative work. Look and feel was dismissed in a US court a while back, and API was thankfully also dismissed. My guess is that the beneficial effect a weaker copyright has on the software market is favored by the courts compared to the arts of films, books and music. That said, only looking at copying as the defining factor is unlikely to yield the correct conclusion. I do think inspeqtor has a decent defense. Since look and feel is not enough for software, Monit will have a uphill battle to claim a narrative that defines inspeqtor as based on Monit. The twitter statement seems to hint towards it, but a court might very likely want more to find inspeqtor guilty.