Vaughters says he had JTL in on March 28th though, which pre-dates when the supposed Sky confirmation came.
The main concern is that JTL was unable to make a date in his calendar, knowing Garmin were interested in him, at a convenient time. For example, he was in the Vuelta a Murcía at the start of March (finishing 2nd on GC), and did not race for over two weeks after this, before going to Settimana Coppi e Bartali where he was utterly anonymous (a pattern that then continued until the Tour Alsace when he was back to his best). He thus tested for Garmin a few days after his DNF in Coppi e Bartali, but couldn't make room for what could feasibly have been, at the time, a once-in-a-lifetime opportunity to get a ticket to the top level, while he knew they were interested in him and he was in the vicinity. Murcía to Girona is a long drive, but if I had the potential to triple, quadruple or whatever my salary, I'd be prepared to jump in the car.
At the very least, doing a power test and getting Vaughters' blessing (Sky used Garmin's power data and didn't suspect anything was up, so they say) fans some of the flames of suspicion when he's negotiating with other teams later.