The French engineers did not build the Berezina bridges. Eble was the commander of the pontonnier units of the Grande Armee. Eble was an artilleryman, and the pontonnier units were part of the artillery arm.
At the Berezina initially, Eble was assigned to build two bridges, the engineers one. The engineers had no equipment so they could not build their third bridge. They, and the sailors with the army, were then assigned to Eble to assist the pontonniers.
And it should be noted that the Berezina bridges broke several times during the crossing and had to be repaired while the crossing had to stop.
Shows how competent the "engineering" skills were in the French army.
Added to that, the French conducted feints at other points along the river to deceive the Russians as to the actual crossing point.
The map of the crossing of the Beresina in Esposito & Elting's Atlas show marshes/swamps about 1/2 mile wide. Didn't stop them building those bridges.