This siutation came up a while back - my opponent drove up a truck into a space already containing a vehicle and offloaded a squad, intending to move it on to another hex, however I opened up with a squad in op fire and pinned the squad as it exited the vehicle. There are now 3 units in the hex (two vehicles and a pinned squad). My opponent had another squad in his truck but my reading of the rules is that he can't exit it as that would break the hex stacking limit, meaning that until he could move on one or more of the units in the hex, the extra squad is trapped in the truck and thus vunerable should the truck being destroyed.
Does that seem correct?