No, you do not need Exchange to use Workplace’s Calendar Sync. This applies to both Savance-Hosted and Customer-Hosted versions.
If you still use the Outlook client (the thick client, not the thin/web client) then you will still be able to use the Workplace Outlook Interface and perform calendar synchronizations with it or the Workplace Desktop Application Interface. This is possible because we do not synchronize directly with Exchange, but rather the Outlook mail profile on the local PC.
If you do not use Outlook, you can still use the Workplace Application or Workplace Browser Interfaces, you just will not be able to sync your calendar events.
The same answer applies if Exchange is hosted externally, such as with Microsoft Office 365.