I think your explanation is likely correct. It seems scientifically sound that momentum would carry the water some time before the level matched what we would typically call a Cat 4 surge. This coupled with the natural funneling effect provided by coastal features like Mobile Bay likely helped to cause a surge higher than what the lay person would have expected. I imagine though that the experts new quite well how much surge was possible and tried to spread the word the best they could. However, it never fails that people think they can ride these things out.