My understanding is a devilish detail remains: A TSO'd txpndr + TSO'd, IFR-legal GPS is still not a legal ADS-B system. The whole system, including the wires, where the antenna is, in a one-of-a-kind EAB plane, has to meet the TSO performance requirements.
Before the FAA's clarification, the ADS-B regs seemingly required a (supplemental) type cert to prove full compliance with the ADS-B 2020 mandate. Since EABs by definition have no type certs, no Bravo, Charlie, or Mode C veil flying allowed.
Now, an (S)TC is not required, just a demonstration your unique, one-of-a-kind EAB aircraft's ADS-B system meets TSO's performance requirements. Apparently, that's still such a complex, expensive thing to accomplish, few EAB owners could do it.
I think this also means you can buy the exact same parts for a certified plane's ADS-B system (even an all-in-one txpndr-GPS-encoder-antenna box as other posters have suggested), have them installed in your EAB by shop with all the qualifications to work on certified ships, and pay the certified prices for the whole shebang...and it wouldn't be recognized by the FAA as a mandate-meeting ADS-B set-up, unless it was tested as a system according to the complex, expensive methods that earned a certified plane its (S)TC.
If I'm not wrong, the Dynon txpndr can continue to be hooked up to Dynon's GPS (or any other GPS device for that matter) after 2020. But the the bits and bytes in the extended squitting will declare "I am not a 2020-compliant ADS-B out broadcaster!!!". You'll still be able to wake up ground stations and get receive ADS-B in traffic, etc. But stray 1 yard inside a Mode C veil and the FAA will probably tell the NTSB's administrative law judge that by gum, they're 100% positive the GPS position was accurate, so the pilot certificate action is justified. <Insert Law & Order Cha-Chunk Sound here>
I believe Dynon has stated on their forum that they have been working with the FAA for quite a while on this, and that it is expected that they will demonstrate that their XPDR connected to the usual suspects of WAAS-enabled GPS (430W, GTN, etc.) are compliant, that they'll provide documentation of that and that the FAA has indicated (or said) that such is expected to suffice for E-ABs. You can search their forum for their exact wording.
This makes the GPS-XPDR system analogous to a XPDR-only system in E-ABs (where there is no such thing as an STC) for installation by E-AB owners/builders to meet the appropriate FARs.
Personally, I think there's an awful lot of hand-wringing over this, along with a pretty fair amount of misinformation (thanks to EAA's Editor-in-Chief, among others).