From everything I've read, Web is 100% correct (as usual). For experimental aircraft, the GPS vendor must state that their product meets the performance required by Part 91.227 (thus meeting the performance required by the TSO). That vendor statement must be supported by testing they've done, but it does NOT need to rise to the level of having received an FAA TSO. As the experimental owner/operator, it falls on you to confirm that the vendor has made such a statement.
Prior to the 2020 mandata taking effect, experimental (and perhaps standard-cert – I'm a little unclear on that one) aircraft could voluntarily be equipped with ADS-B OUT where the GPS did NOT meet the TSO requirements. They would be classified by the FAA as "voluntarily equipped, but non-compliant" with the rule. Until the 2020 mandate goes into effect, their data will be used in the system and re-broadcast to other aircraft. But once the deadline passes, that "non-compliant GPS" data will be suppressed by the FAA, and such aircraft will NOT meet the equipment mandates to fly in "ADS-B-required" airspace.
At one point, it was understood that such "non-compliant ADS-B" equipped aircraft would receive notices requiring them to disable / remove that equipment, or upgrade it to "compliant" status to keep using it. I'm not really sure how that decision played out in the end... Anyone else know?