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.