It looks like they haven't updated that page 🤔 The resources are coming from the Cloud Formation ListTypes API
where each type is marked as “FULLY_MUTABLE” or “IMMUTABLE” if it’s supported in the Cloud Control API (CCAPI). That's the API that's underneath the CCAPI (the part on their page that talks about anything that's in the CloudFormation Registry is really talking about how CCAPI hits this API for those types). We generate that file with this code
, for reference.