The leaner you are, the faster you will gain muscle when put on the right program. A lot of skinny people would bulk up far faster than prime Schwarzenegger could ever dream of.
9 kilos in 8 months is quite a lot, but not unrealistic on an ambitious program if he has the genetic disposition for it (crouch for example, simply could not gain weight however he tried). He'd have to be eating truckloads of food to compensate for his cardiac activity and calorie burn, but I wouldn't call it impossible.
If it had said 6 kilos in that time period, I wouldn't bat an eyelid, tbh.