The rulebook is a bit contradictory. The bit on page 41 certainly suggests you can't ambush within optimum range, but then the text and examples on page 42 suggest you can if the enemy fails a spot check on your hidden unit. I leave it up to the AAG staff to clarify.
Actually I think it’s just a question of timing. My interpretation is that a unit cannot prepare/set an ambush if it is already within the optimum range of an enemy unit, on the other hand if there is no enemy unit already within optimum range it can spring an ambush against enemy units moving within its own ambush range.
Case 1: A non initiative Russian infantry unit (TQ8), hidden within a terrain feature tries to set an ambush against some US Rangers (TQ10). Unfortunately for them a Ranger unit is already at 10” from their position, therefore the Russians cannot spring an ambush.
Case 2: A non initiative Russian unit (TQ8), hidden within a terrain feature tries to set an ambush against some US Rangers (TQ10). BUT this time there are no Rangers within 10”. So when a US unit comes within ambush range the Russians will be able to string their ambush.