Just because you were taught to deice when there is snow on the wings, does not make you correct.
This crew wants to live as much as you do. You really think they came up with this method out of thin air?
The crew had probably done it hundreds of times, and got away with it. However, one in several hundred chance of a disaster is not nearly short enough odds for for commercial air transport operations (in the West), where odds in one in a million are deemed more acceptable.