Brilliant move by Schumacher. And according to my knowledge of the rules, absolutely legal. The rules are clear: for this year onwards, you can race as soon as you pass the pit lane line. On a restart, if the green flags are shown they are allowed to race, even if it is the final lap. The rule where you have to stay in same position only applies if the flags that are shown are yellow.
All drivers realized that the marshals were showing green flags, and all accelerated to line like it was a normal restart in race conditions.
Even on the official timing system, controlled by the race director, it was shown the green flag.