Gary's answer:
For the pro-Caesar aspect, it makes sense in the other interpretation as well.
Caesar's horsemen were beaten. His camp was overrun. The enemy was
simultaneously in the camp and the woods and at the river. Everything looked
hopeless for the good guys. Then, in the last paragraph, Caesar comes out
and saves the day, simultaneously raising the banners, calling the troops,
blowing the trumpets, etc. What better way to convince the folks back home
that Caesar himself was indispensible to the res publica?
Answer by Tim Haas (Timothy or Lillian Haas [spooner@free-market.net]):
I think in context it can only be Caesar's cavalry who were routed. It was
they who dared to pursue only to the edge of the woods (presumably because
their numbers were insufficient to engage the enemy in unfamiliar
territory). Then the enemy, seeing that the legions were otherwise occupied,
rushed out with all of their troops (omnibus cum copiis), overran Caesar's
horsemen, and rushed to the river (hence Caesar's comment about them being
everywhere at once).
An old interlinear translation I have here confirms this theory ("suddenly
they flew forth with all (their) forces, and made an attack upon our
horsemen. These having been routed easily and disordered, they ran down with
incredible speed to the river; so that almost at one time the enemy were
seen both at the woods and in the river, and now in our hands [close at
hand]").