If you have your own domain name, you could create a separate Namespace for each site.
For example:
www.example.comdev.example.com
The dev domain could be a CNAME which points at the deploy-preview-<some number>-catsource.netlify.com record.
Alternatively you could create a separate Namespace for deploy-preview-<some number>-catsource.netlify.com
Either way, each domain/site will need it's own Namespace and separate code snippet.
I hope this helps and finds you.
I know this, but why was I able to comment on the deploy preview domain with the code for the "normal" site?
An invalid rb_owner_key in the snippet will still allow commenting on the site to work, but it won't unlock moderation abilities for a user until the code matches on both your website and on the back end. Each domain in Remarkbox is expected to have it's own Namespace (and the first step to that is each need their own rb_owner_key which happens when you click setup and follow the prompts).
Remarkbox