InsideEV usually doesn't disclose their method, so I can see why you would try to make that argument. But in this specific case, they subtracted their Jan/Feb from Tesla's official announcement to get a March estimate (linked below, thanks to
@mongo who dug it up). TMC has been predicting the Q1 #s (delivery, production, and exit rate) spot on starting around mid March. IMO TMC is showing much stronger predicative capability than InsideEV. In fact, "lol this is awesome" is my exact reaction when I saw InsideEC's April #.