Caz,
The Boards conclusions were based on a weather "forecast". What exactly the crew encountered will never be known. With a "forecast" iceing level of around 3000 ft, actual iceing can occur anywhere from the surface to infinity or never.
The Board "speculated", "best guessed", call it what you like.
You and I both know, that what you are forecast to get at a particular time and place and what you "actually" ( that word again) encounter are rarely the same. The only people who knew the conditons at the time and place where a climb (probably?) should have taken place can no longer tell us.