Glide angle estimates of 16/18:1 are presumably for a 'clean' airframe. The pictures, after removal from river, show LE slats out and some flap out on sbd wing (but not obviously on port wing so could be post splash damage) and wheels up.
What would glide angles be in various 'dirty' configurations?
As a glider pilot, ...... clean makes a hell of a big difference to best distance, but so does speed, and I would be prepared to bet that an A320 would need to be much faster than climb out speed to have best glide distance/ratio and no way these guys would have traded height for speed .
Also - As a pilot that does regular "off field landings" - you obviously try to be as slow as you can over the ground when landing, so once the decision has been made to land , you use whatever slow down devices you have (airbrakes and flaps on most gliders) - thus the most likely deployment of slats/ flaps here, and they were probably only deployed at last possible moment.
I think we all would like to know if the APU was still running as that would have made a big difference during the emergency.