Does iCloud sync calendars even when iCloud storage is full?

Wanted to use the CalDAV for a project, however didn't want to upgrade my storage unless required. When I added a test event on my Macbook, It immediately appeared on both iCloud.com and my iPhone, even though my iCloud is full. Is calendar data considered negligible and syncs even when there is no iCloud space available, or is this some local, over-the-air sync given my devices are in close proximity to one another?