Interavls.icu will delete the Strava activities if there is a matching activity from another source. So you need to undelete the Strava activities and then delete the Garmin ones. You can do this on the activity list view.
Select the correct date range and filter for deleted activities. You can add activity source as one of the columns:
Click the date column to make sure they are sorted oldest first. Then choose undelete.
You can do something similar to delete the Garmin ones once all your Strava stuff is back. But be careful because it is not easy to undelete Garmin stuff. They strictly limit calls to fetch old data.