I've had inaccurate logs of my flights from flightaware before. And I know they were inaccurate because I made the flight plan and flew it and logged my times. It is not frequent, but more common than it ought to be. But would be one helluva coincidence that on this day at this time, that particular medical helicopter had an inaccurate log of the departure time to Uvalde...that's a trifecta of bad luck right there.

One other possibility - we don't know if the hospital needed to transfer a patient unrelated to this incident, nor do we know if there was a serious car crash necessitating air lift.

I don't know what standard ops are for a medevac helicopter, but I would presume that time would be of the essence in departing, so no time to file an instrument flight plan. Get a VFR clearance to take off and go. How you'd have a scheduled departure time absent an IFR clearance seems not possible.