Kevin,
CS Forester, as you say, wrote 'The Age of Fighting Sail' and the Hornblower novels as well as numerous other excellent works of fiction, some of them set in the Peninsula. Being an expert on the Navy doesn't make him an expert on the French army and the passage you quoted shows all the signs of being based on the received wisdom concerning Bernadotte rather than any personal investigation. No-one can be an expert on all points.
Ramsay Phipps, whom I greatly admire as a historian, was not without his prejudices.
When it comes to Bernadotte, he has had such a bad press in the past (OK, much of it deserved) that I would suggest that the incident under discussion should be considered independantly of past and previous allegations.

regards,

Susan