Plenty of other cold blooded snakes and animals survive the Winter in Northern states. Also, if you believe that this particular snake could not do so, then we would have to assume that someone dropped a 20 foot snake into the lake just this Spring. As unlikely as this whole story seems, that seems even more improbable.
Yes, cold blooded snakes do survive in the northeast, because they are able to hibernate. Giant tropical snakes don't do that.
I have kept many pet snakes over the past five decades, including boas and ball pythons. Anyone who has ever kept a tropical snake as a pet knows that keeping their environments warm enough is a critical aspect of their care. In all cases, that minimum temperature is 75 degrees.
Moreover, it is completely plausible that someone dropped a 20 foot snake into an area near the lake, and probably fairly recently given the average temperatures in the Northeast through this past April.
Many (stupid) people keep large snakes and then decide to release them in the wild when they get too big and too expensive to maintain. Or possibly, it escaped.
It is absolutely impossible that a large constrictor survived this past Northeastern winter (or any other winter) in the wild.