Ian, that's correct. The space saving in the mailbox is just the removal of the attachments
Calendar items are definitely eligible for archiving. The change is that we no longer convert items into a standard EV shortcut as that would mean that the calendar item no longer behaves as a calendar item in Outlook
When calendar items are archived the item still appears to be the same, but if it has attachments then they are stripped from the Outlook copy and retained in the archive instead. There is a policy setting in Policies > Exchange > Mailbox > policy > Advanced which controls the stripping behaviour (Strip attachments to non-shortcut items)
Note that calendar items that occur in the future or have not expired (recurring appointments) are not archived by default. Policy settings also apply for handling those items
