Very difficult decision, whichever way it's viewed.
1. Yes, he was actually moving forward very slightly, but as I read on Twitter the right hand was holding the throttle wide open, and his left was holding the clutch in while his left thumb was on the rear brake lever to try and stop it creeping forwards - how good is your coordination, to do all this while watching the lights, adrenal glands flowing at 100% and 22 other bikes revving at full tilt all around?
2. Did the few cm his wheel crept forward (without crossing the line of his grid marker) give him any advantage for the race?

Unfortunately the rule book is a very binary object; technically Cal was creeping forwards, but a negligible distance, no advantage gained, but the rule book says that a jump start has to be punished with a pit-lane ride through. If a rider completely leaves his grid spot then fair enough, but maybe the rules need revising to add the proviso that without leaving the grid spot, pre-movement merits taking the penalty lane (at turn 7 on this circuit), a penalty of 4 or 5 seconds, more proportional to the scale of the offence than the 25 seconds that this penalty cost him. Given that Cal's pace was often second only to that of MM93, the long lap penalty could have ended with him finishing in the top 6, if not on the podium... It's probably too late to do anything about it now, and it would be tough on Cal if they were to change the rules once the season is underway without making them retrospective, and how would they mitigate it to his situation - give back 20 of the 25 seconds that he lost? That would at least have given him 4th position overall...