It's one thing if a tool designed for legitimate archiving use results in incidental violation of copyright.
It's another thing entirely, legally, if a tool is designed for violating copyright. In such case, it doesn't matter if there are also legitimate fair use uses of the tool.
And that's what the README showed: that youtube-dl was built primarily to violate copyright.
EDIT: based on other comments, it turns out the offending evidence was in the unit testing code not the README file, which legally makes it a bit worse than if it had just been in the README file.
It's one thing if a tool designed for legitimate archiving use results in incidental violation of copyright.
It's another thing entirely, legally, if a tool is designed for violating copyright. In such case, it doesn't matter if there are also legitimate fair use uses of the tool.
And that's what the README showed: that youtube-dl was built primarily to violate copyright.
EDIT: based on other comments, it turns out the offending evidence was in the unit testing code not the README file, which legally makes it a bit worse than if it had just been in the README file.