Many TP companies use an acceleration altitude above what was achieved in this accident but my ATR buddy tells me that the ATR manufacturer's procedure is to retract flaps at 400' agl. Given the time to call it, make the selection and for the flaps to travel, the maximum of achieved 680' could tie in with the result of no flaps on an iced-up wing.
If everybody else was de-icing that morning then so should have the ATR but reports do indicate it was not. The captain is 100% responsible for the de-ice/no de-ice decision as as for the suggestion de-icing may not have been available, that's no excuse you just don't take-off - simples!
From what has been said so far it does sound like a loss of control with a low speed/high descent rate impact. Lots of conjecture and no surviving crew so we'll have to wait for the FDR/CVR to tell us what really happened.