Not Adding Power;
Obviously we won't know this is the case until an official report is released. But it sounds plausible to me, how else did the A/C end up so slow?
When working as a FO, I've seen captains who have lot's of experience on other types do this a few times when they are new to an A/C. Especially if the A/C is configured with gear and flaps out. With a few other factors in the mix, it really isn't hard to only add power that would be appropriate for a clean configuration when leveling off.
I know of a case recently where the captain didn't even respond to the two challenge policy, and the first officer took control while the stick shaker went off. This was during the step down portion of an approach. That captain had +7000 hours on turboprops. I'm sure people make this mistake a lot more than we suspect.
Possibility of Severe Icing;
Could have easily happened. Forecasting icing isn't an exact science. But that A/C, at that altitude should have been able to handle almost any icing if the appropriate speed was maintained.
Good post Beafis