Antcufaalb covered this nicely at some point. If you had, say, a diff file that, when applied to the legal retail product, produced the fanedit product, could you distribute the diff file instead of the fanedit?

The answer is no. His example was John Cage’s 4’33", which is 4 minutes and 33 seconds of dead silence. If you distribute a track that’s 2 minutes of dead silence, is that copyright infringement? Legally, it depends. If it was 2 minutes of your own silence, you’re good. If it was 2 minutes recorded from John Cage’s 4’33", then you’re not. Exact same files, bit-for-bit identical, but one is clear and one is not. The difference, legally, is that one was derived from copyrighted bits. Back to the original example, since the diff tool was created directly or indirectly from the copyrighted work, it too can be considered covered by the copyright.