From the article:

Spaces, a new protocol primitive, provide a way to store and sync non-public data while retaining the advantages of atproto like portable identity, interoperable/remixable data, and permissionless participation.

Today, we’re making the alpha available with running code, published SDKs, a sample app, and even a hosted PDS you can create an account on and develop against. This is truly an alpha. There will be breaking changes, and you absolutely should not run production code against it.