This is a test implementation of a course-specific Google-Calendar, which is synced to the course calendar in OCA Learn. The OCA Learn Course calendar is in turn synced to a Course-specific Google Calendar, so whether events are created in the course on OCA Learn, or through the Google Calendar, they are visible to students through the embedded Google Calendar (as below.)
The core code has had to be hacked, as otherwise no link to the OCA Learn activity is made available in the Google-based calendar. This is now fixed, but will need to be carefully re-applied along with all other core code changes following every core code update; approx two every year.
Second issue, which probably can't be resolved, is that by syncing in both directions, events created directly through the Google Calendar, by being synced to OCA Learn, results in that same event being re-synced back again to Google, so duplicates of those events appear.