Typically ppl like to have the distance / Elevation frm their Zwift rides into Intervals.
Having said that, are you dual recording? Ppl also dual record to get IQ Data from their headunits popolated as well (eg: CORE Sensors etc)
also, from what I can remember, (BreakAway: Indoor Training - Uploading Completed Workouts to Garmin Connect) Rides from Zwift, that gets loaded into Garmin will be sent into a Garmin HeadUnit / Watch etc (to calc Vo2Max, Load etc) which will then round-trip back to Garmin Connect.
you can verify this by looking at your wtach/headunit History. You can find your Zwift ride in there as well.