This is wrong, because they didn't make a 512 claim. If you read the letter, the only part that's asserted under penalty of perjury is that they're authorized to act in behalf of their member companies, which is true.
> RIAA are not injured due to utilisation of a non-member's mechanism.
They're injured when the tool is used for copyright infringement. No issue with standing here.
>Any potential copyright infringement which might occur through use of youtube-dl is at the volition of users, not the software's authors, actions would properly be directed at such users for individual acts of infringement
This ignores the fact that circumvention itself creates liability in the right circumstances.