The River Coa rule applies only to the bridge hex. The rule in question would mean that the 6 French unis on the baseline could never move. And that is just this scenario.
The rule in question states that no unit may move off the baseline unless explicitly allowed by the Battle Notes. Since all scenarios have some units set up on at least one of the baselines, and there are no rules in the battle notes except for the one hex mentioned, then either no unit set up on the baseline in any scenario, or retreated to any baseline hex would be able to move.
This is an obvious mistake, and it should be noted.
I believe this means no units can exit the board from the baseline. Not that they are permanently pinned down from moving forward. This is a common rule in the C&C games. But some scenarios across the different specifically allow units moving off the board on the opponent's baseline.
The rule you quote doesn't say "baseline hex" but instead "baseline edge."