The routing of an entire Roman army is as much a yarn as anything. A more likely scenario was a deal was cut.
How so? The no army is ever a stranger to defeat and Rome has witnessed many near catastrophic defeats where entire legions were destroyed. Hannibal, Sparticus, Alaric, do the names ring a bell?
Here are significant battles where the Romans lost entire armies in significant battles. The defeat of an "entire Roman army" happened a few times in their otherwise glorious military history:
1) Battle of the Allia
2) Caudine Forks
3) Battle of Cannae
4) Arausio
5) Battle of Carrhae
6) Teutoburg Forest
7) Battle of Adrianople
8) Alaric's Attack on Rome