Avidyne v. Garmin Data

Friends:  In a discussion about the best touch-screen navigators on the Cessna Pilots Association online forums, I lamented that Garmin won't allow my Avidyne IFD540 (or indeed any non-Garmin box) to provide an approved position source for my GTX-330ES transponder.  I concluded as follows:

“In a perfect world, boxes from different companies would accept each other's data streams freely. But that's not the world we live in; Garmin radios are happy to output data to other avionics, but they won't accept data from others. On the other hand, I believe that Avidyne avionics will freely accept data from other companies boxes.”

One of the other guys on the CPA Forum responded thus:

“I believe that Avidyne wants you to believe they are more open, but it doesn't appear to be that way. As an example, they claim to use the Capstone ADS-B protocol so that they'll be open to other devices, but they have yet to claim they are compatible with any other brand, even those that support the capstone protocol. They also haven't stated that they'll allow their IFD540 to be used as an approved GPS source for another brand's ADS-B box. They have their transponder and their recently announced MLB-100 that they want to fill the ADS-B role so conveniently they haven't claimed to be compatible with anybody else.

For what it's worth, I'm an IFD540 pre-purchaser so I really wish they were more open. It just seems like they want to sound like they are more open, but in the end they are going to play the game the same as Garmin.”

Is there any truth to these allegations?  If not, I’d appreciate your help providing an informed rebuttal.

Thanks, Mike