That is correct.  We've contacted Garmin in an attempt to connect with the GDL-39 via hard-wire.  Garmin noted that they will only integrate and connect with other Garmin devices or Garmin apps.   I stressed to them, that it would be a nice safety benefit for GDL-39 owners and prospective buyers to be able to connect to their other non-Garmin devices.   They commented they will look at it for the future. 
Please know that it is our desire to maintain the most compatible ADS-B device on the market.  Currently we support, Clarity, iLevil, SkyRadar, NavWorx, and are currently working on FreeFlight, EFT, and others.  I hope this helps.