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.
The name on that email is not one of the Runbox folks listed on their About page, so one can only guess who that actually is or how the email was sent; it could have been a BCC for all we know. https://runbox.com/about/runbox-team/
It's the same idea behind a VPS provider serving their status page on another company's infrastructure.