Yes, in fact it can be faster then that.
Here's a good example from my own games.
Broodlord and a squad of Genestealers assault a rhino that moved 12" last turn.
At initiative 7, the broodlord goes first, and scores a single hit thanks to only hitting on 6's. He glances and ends up immobilizing the rhino.
At initiative 6, the rest of the brood goes next, and they all auto-hit due to the vehicle being immobilized.
Doesn't seem very realistic, I agree, but 'dems da rules'.
BRB pg 63 Table in left hand column "Attacking a vehicle that is immobilised or was stationary in its previous turn >>> Automattic hit"
The only times when you get an automatic hit are:
1. When a vehicle is immobile
2. When a vehicle has not moved in it's movement phase (pivoting does not count as moving)
3. When a vehicle has been immobilised in another (i.e. not this one) turn (firend or foe) and is still immobilised or hasn't moved (i.e. repairs)
1. Drop pod lands in players turn. Opponent assaults, as it is now immobile; automatic hits.
2. Vehicle does move in players turn. Opponent assaults vehicle; automatic hits.
3. Vehicle fails dangerous terrain test and becomes immobilised in players turn. Opponents assaults; automatic hits.
4. Player shoots vehicle and causes immobilised result. Player assaults vehicle; NOT automatic hits. The vehicle was not immobilised in the previous turn.
5. Player assaults vehicle with different initiative values. Highest causes immobilised, lower initiatives assault; NOT automatic hits. The vehicle was not immobilised in the previous turn.
Hope this helps.