The leading edges of the T-tail also has pneumatic de-ice boots on the Q400, in addition to those on the wing leading edges and engine intakes.
As has been said, the apparent wx conditions well within the capabilities of the 400's de-ice system provided it was operating normally. From what I recall, the system needs to be selected manually to 'ON', and once selected runs in an automatic cycle. Detection of icing conditions determined by an ice detection system (message displayed on centre EFIS screen alongside engine instruments), with additional visual cues coming from the ice detection spigot on the wiper arms, and from visual inspection of approximately 2.5m of the wings visible from the flight deck.
However, the de-ice system on the 400 is prone to minor failures (with a caution illuminating on the master warning/caution panel), requiring crews to exit & avoid icing conditions. Had this been the case, I would expect the crew would have had ample time and opportunity to alert ATC should that have been necessary.
As for the 'chainsaw' sound reported by one witness, I doubt it would be the PTU. It's operation is fairly hard to detect on board the a/c, let alone as a ground-based observer.