“Is it possible there was an altimeter error on the chopper?”
I don’t know know if a radar altimeter — which wouldn’t be subject to much error — is in use on a military aircraft when operated in civilian airspace like that.
But regardless, helicopter pilots develop quite a keen sense for altitude above ground even at night (out of a sense of survival). And there were plenty of lights along the river. If the 200’ AGL ceiling I read is valid, it’s hard to understand how he/she could have been off by a big fraction of that. But he evidently was.
All military helos have radar altimeters. Unfortunately at night over water, there is no such thing as good depth perception until it’s too late. More than one helo has bounced off the water due to inattentive pilots.