> "The plane had been cruising at 17,000 feet until 1:21 p.m., when it dropped approximately 250 feet in 10 seconds. It then climbed approximately 400 feet in about eight seconds."

> The last sentence suggests that they were actively trying to get back to their assigned altitude of 17,000′.

I believe the author is implying that if the pilots attempted to maintain speed rather than altitude, the aerodynamic stall wouldn't have occurred - if they descended below 12000', the icing would be reversible.