The update checker only looks for stable versions. (Stable vs Beta versions, and the automatic update checker)
The list of news is the list of posts to the Opus News blog, which includes a post updated for 12.0.6 (2nd in the list). The news posts don't drive the update checker; that's a separate mechanism, but shown in the same dialog.
Beta updates are downloaded and installed the same way you got the beta originally. Just download the installer via one of the beta announcements and run it.