The platform/service is still in beta and opening up invites to its first users now.
The main developer is on mastodon: https://mastodon.social/@reiver
Their webpage if you want to signup for early access: https://spacehost.live/
From what I’ve gathered, they intend to support a variety of fediverse platforms, but have started with lemmy with kbin close behind.
Is the platform open source?
No idea actually. You could ask them on mastodon. I’d bet not, or at least not yet.
Asked.
Let us know if you get a reply?