In other words: that argument is interesting, but it feels strained to me :-) -- I don't think RubyGems or Ruby Central is actually legally liable in this way (or if they are, it suggests a failure of clarity in their EULA/TOS).
The (mostly PR) explanation they produced seems to express roughly the same thing I was guessing though: https://rubycentral.org/news/strengthening-the-stewardship-o...