A tarball of _ancient_ DeCSS code has some value because you can use it to "decss" _all_ content made _before_ a certain date (e.g. most physical DVDs). So it makes sense to preserve the tarball in any method possible.
A tarball of an _ancient_ youtube-dl version is absolutely useless because the youtube HTML will have changed a million times in the meanwhile. You will not be able to use it for anything, neither for old nor for new content. Publishing/preserving youtube-dl tarballs is an absolute waste of effort.
The RIAA here is targeting existing project developers, and possibly also users. Trying to scare them away. Not trying to censorship old tarballs of the code which will quickly become useless.