Ansom doesn't have to start moving himself + Jillian away from column until AFTER the 3 dwagons are dead. There is absolutely no need for him to be near that fight when it happens. He can move himself when he feels like it during his turn. So if he pokes the central hex with a single bat prior to committing himself into battle, he'll know its a trap/empty hex. In that case he'll have a still rather powerful group ready to pounce the hiding stack. Then he'll simply need to scout a large area with bats to find the hidden wounded dwagons and wipe them out. Hopefully he wont have that much foresight and/or moves. Otherwise he can still turn this into a victory.
EDIT: But yes, the forest walkers are doomed. They'll get sacrificed fighting the 3 dwagons with no way to get back by the end of their turn.