I would hardly call this a simple slip-up. It is known that the software could be used to infringe copyrights and other open source projects make it clear that they do not support copyright infringement even when it is clear that their software can be used to do so. (If I recall correctly, Kodi and Calibre are two examples.) Meanwhile, youtube-dl has a test for that specific use case.
I feel that is rather thin legal grounds, if the examples are what dictate legality. The fact that I can change some text that does not change the function of code, or what can be done with it, to suddenly make it more legal does not sound like very great legal basis.
I don't know whether I agree that it should be thin legal grounds for a couple of reasons, even though I do agree that changing a few lines of text that doesn't alter the function of the code makes the argument sound sketchy.
Here's why I am not sure whether I agree:
The text in question is described as being part of a test. It is highly suggestive that at least one developer wanted to consider cases where the rights holder may be more assertive (e.g. with potential protection mechanisms), which implies intent. Granted, I don't know what the actual intent was. Only the people who added that text and accepted the commit can know that.
Intent is important when laws are broken. In the minor instances, it may determine whether the law is even enforced. In more significant violations, it may be considered when evaluating legal recourse or damages awarded. While I am uncomfortable with the law being fuzzy, I am also uncomfortable with it being absolute. The latter is especially true when you consider that it is far too common to create laws that protect singular interests.
