Not sure what "first class" and "support" really means. The announcement text suggests they just install the main Nim distribution. Still, it's nice to see an announcement.
It perhaps bears noting that Gentoo has had a dev-lang/nim ebuild in their official repository for like 6+ years. I think it kind of pre-dates the git-ification of their process in 2015 and my recollection is that there was an ebuild around in 2014.
Nim is provided by many distributions at this time. Repology provides a (somewhat simplistic) overview: https://repology.org/project/nim/badges
Debian (and its derivatives) provides Nim since 2015 and a handful of libraries. They can be listed with apt-cache rdepends nim