It doesn't look like that email actually belongs to Fastmail staff, but if it did, having a backup on another email service is a good idea in case there's a DNS

It doesn't look like that email actually belongs to Fastmail staff, but if it did, having a backup on another email service is a good idea in case there's a DNS issue with Fastmail or something. It's the same idea behind a VPS provider serving their status page on another company's infrastructure.