If established at best L/D when they took manual control I agree but the autopilot from what I remember was holding the glide slope so they were well under L/D and a few hundred feet off the ground they would have had to lower the nose and greatly increase their sink rate to regain it. In Sully's case he had the altitude from 3,000 feet to do that. In this case the engine problems happened at 700 ft so they had to be well below 500 ft before they disconnected the AP. I'm sure when this happened is well documented in a few thousand posts back.