This is not that vulnerability. Their Rails is just fine. This is related to the fact that gems use YAML to store their metadata.
Gems, specifically should be signed. They are not, this type of exploit will continue to happen, hell, remember when github screwed up ssh keys? Who knows what's in the ecosystem.
TL;DR; Ruby's security ecosystem is butter.
Disclaimer: I love ruby and use it daily. It was two critical problems IMO: unsigned code and GIL. Yes, GIL. I'm looking at you ruby-core.
(This isn't a new technique -- for example, .deb packages distributed through APT are usually signed with gpg -- IIRC, this was a measure introduced years ago in response to a Debian mirror being compromised.)
Debian has (had?) a high barrier to entry to become a developer, and every developer signs their packages. The release binaries are arranged on a secured box and the release key itself is held by a limited set of people.
In short, the signatures work because of the human element and organizational structure of Debian.
Rubygems accepts submissions from the general public.
So, again, I don't see how it would have helped.