Nitpick: They would still fail for DNS issues with *.github.com, so a domain like githubstatus.com would be even more resilient.
Nitpick: www.githubstatus.com is more flexible, potentially more resilient.
Nitpick: www.statushub.com is less obviously related, and therefore won't be attacked in tandem. If they want to go all the way, maybe just something like www.wesellnikesdiscount.com.
Were they doing this before amazon had that major outage?
Yes. They've been hosting it outside of their production infrastructure for several years.