Yeah they’re completely and totally different things. Fly.io is actually hosting servers / services, while MRSK is basically a bunch of scripts to deploy things using Docker containers.
It’s like comparing AWS with Chef. Maybe there’s some overlap in philosophy, but that’s about it. Maybe fly.io is just using it as a way to do a bit of marketing.
A head to head comparison makes some amount of sense. Fly.io is a platform-as-a-service provider, and natively handles stuff like like building/storing images and orchestrating your services across their DC.
MRSK is a direct competitor to Fly's own orchestrator, and is meant for use when hosting on your own hardware or on a VM rented from an infrastructure provider. So it's still a "use either A or B" situation.
I guess a better title would be – Hosting on the Fly.io platform vs. on your own servers/VMs with MRSK.
It’s like comparing AWS with Chef. Maybe there’s some overlap in philosophy, but that’s about it. Maybe fly.io is just using it as a way to do a bit of marketing.