That’s correct. The Garmin 5-year history sync to TrainingPeaks is for Garmin recorded workouts only. TR, Zwift, Rouvy, etc, despite being in GC, will not send to TrainingPeaks. There is no way to force it. Garmin controls what they send and have chosen to only send Garmin recorded workouts only.
You would need to bulk export your TR history from TR, then click, drag, and drop into TP