On the actual C3i magazine scenario (I own that issue of magazine) it is described differently:
"To simulate the Alamanni ambush, when an “Order Light Troops” Command card is played by Alamanni player, place two light infantry units from off board, adjacent to any Roman cavalry units on the Roman right section. These two light infantry units are ordered in addition to number of Light units ordered by the “Order Light Troops” Command card. Once these two light infantry units are placed the function like light infantry for the rest of the game"
The Romans advanced well and took the central position while on their right the cavalry pushed the Alamanni, scoring 9-4, but the barbarians started launching warrior units that broke the Roman line, on the Roman left the woods stopped the advance and on the Roman right the barbarians started defeating the cavalry. Finally, with well planned moves, the Alamanni scored enough to win a close 12-11.