The post says "hardware isolated" but below in the sandbox it says firecracker, which I thought were supposed to be a secure way to run containers from multiple tenants on a single host. Also I thought Fly machines were already using firecracker.

I'm having trouble understanding the difference to Fly machines. If you spin up a Debian container on a machine with a persistent volume, doesn't that have everything this does? Is this about providing a layer of useful configuration/management software on top?

Subtle to explain. I'll explain better later this week. For now though, just know: every Sprite is under the hood a KVM VM.