At the risk of being an armchair judge......
Due to the helicopter's airspeed, he was not sure he could make it to the East River and reduced rotor rpm so he could "glide better."
he began the cyclic flare in an extended glide configuration, but he "did not get a lot of rpm back.
Hate to break it to him but the only way to improve your glide range in auto is to increase your speed and
NOT decay Nr. But it seems he didn't have the height for that.
Bleeding off Nr with a duff engine is going to place you behind the drag curve when the time comes to recover it.
Looking at the vid again, suggests to me that the last 100 to 50 feet brought home to the pilot the realisation that his Nr was not high enough to arrest the RoD enough to minimise impact.
That very (early stage) action (drooping Nr) may have been the last straw for determining the submersion of the cabin immediately after impact.
Lots of Nr is your ONLY friend during an EOL. And a zero/zero may well have resulted in a stable and upright cabin platform on a relatively calm surface (Notwithstanding assymetric float deployment). Giving time for an evacuation by the odd pax?
Fuel: ON.
Ignition: ON.
START button.