With iBackup Viewer you can export events from all calendars, or just single calendars extracted from iPhone backup.
Click the "Calendar" icon after opening a backup to load calendars and events.
Click on the "Export" button to show options
Choose 1 "Selected Calendar to ICS..." to export all events of current calendar to a single iCalendar file.
Choose 2 "All calendars to ICS..." to save all events of all calendars to separate ICS files.
Click on individual event will show details about the event, click "Export" button to save the event to ICS file.