Indeed, green flag is for the sector following the incident, not the incident itself. It is also worth bearing in mind that JBi had already driven past the scene one lap previously under waved yellows and would have been visually aware of an ongoing situation.
He was also on worn inters at a time when JBn and Maldonado had pitted for full wets, so either he and/or the team were rolling the dice or were slow to react. Either way, it is a consequence of a motor race. JBi injuries have been described as as a result of massive deacceleration so it is a moot point whether the presence of a tractor, another car or the Armco would have made much of a difference, as long as there was regrettably some kind of solid object in his path.
Hopefully he will in time, make a full recovery.
On the plus side, Jean Todt reports better news for Schumacher, saying he thinks Michael will be able to function "quite normally".