the last post on that Garmin forum was over two years ago. Actually Garmin can now sync swim workouts. I do it every day from Training Peaks. Here is info
Structured Swim Workouts FAQ
Agree. It is now supported out of Garmin.
I’ve used other training platforms (Humango) and they synced swim workouts.
Btw am trying Athletica because I wasn’t seeing much improvement in Humango.