Just for info, this is what the FAQ linked above says on Page 52:
Q. Does the Ambush card allow the Ambushing unit to advance if the target is forced to retreat or is eliminated?
A. No. Only when an ordered unit in Close Assault Combat eliminates the enemy unit or forces it to retreat, may it Take Ground.
The key word here is ordered. Only the player taking his turn (ordering units) may Take Ground during his turn. The Ambush card
is a reaction play when an opponent declares a Close Assault, and therefore the unit doing the Ambush may not Take Ground if
the opposition is eliminated or retreats.