Gentoo's wiki is still great (& Arch's has been great for a long time), but yes, Arch's is probably improving at a faster rate. Arch is also a little more comprehensive when it comes to mainstream tech that's divergent like init & network management - Gentoo's still good here but openrc & netifrc show their influence throughout.
I think the Gentoo wiki that gp is referring to is the old gentoo-wiki.info that went down after a hardware failure or something like that and never came back.
The wiki.gentoo.org we have now restores some of that but probably not everything - and there was a void left between them that let the Arch wiki gain mindshare.
