Under semver (which isn't the only versioning scheme, but certainly among the more common of the ones that have a firm definition of what 1.0 means), it means "stable API", which doesn't necessarily mean "production ready".
Version numbers don't have on single universally accepted set of defined semantics.
If I remember correctly, the RethinkDB team opted to match their in-house version numbers with public releases. So the 1.x releases would be more comparable to 0.x releases for other projects, and 2.0 would be considered stable and production ready.
1.0 is supposed to mean production-ready. Are you suggesting this should be a 0.7 release?