and I just read this info on the zwift connector page:
You can’t download historical activities from Zwift (yet?) and you need to make sure your FTP on Zwift matches your FTP on Intervals.icu. I am hoping Zwift will extend their API to support settings sync.
You got it all correct.
Garmin will not forward activities not recorded on their hardware. And Zwift will only push new ones at the time of ending them, not historical ones.