I used a variety of GNU/Linux distros: Ubuntu, Debian, and Arch, Slackware, Frugalware, Fedora and more recently openSUSE Tumbleweed.
Honestly, I don't understand what makes Arch so special.
I have mostly learned things on Ubuntu and I can use Arch just fine, and I think I have a decent understanding how things work.
Really, I think there is nothing fundamentally different across these distributions technically speaking.
Their biggest differences lie in their package and release management and policies (and, agreed, this is huge).
I'm not sure what skill you would learn on Arch and not on Ubuntu. It seems some vocal Arch users are lying to themselves and to the rest of the world about this.
Gentoo (or LFS) might probably be more educational, but I haven't used Gentoo enough, and haven't tried LFS enough to say.
(Kudos for Arch's documentation though, useful even when using another distro)