I refuse to believe that this crew did not have the boots turned on at this time.
Let's take in the facts:
The Q400 is a new aircraft with vairous protection systems. It is not some 1970s diesel burning puddle jumper. That being the case, there are many levels of protection in anti-ice systems. While I am not familiar with this specific model, I do fly aircraft from this manufacturer, and therefore some basic systems will be similiar.
To prevent any speculation at all, let me use my aircraft as an example (which is of the same age and manufacturer as the Q400). Anytime the engines are running and everything is set for flight, if I were to turn the left cowl anti-ce on (heats the leading edge of the engine cowl to prevent ice from forming and breaking off into the fan blades, we'll just call it cowl), I get a green advisory message that says "L COWL ANTIICE ON." If I were to then turn on the other engine's cowl, the system recognizes that fact and the previous message changes to "COWL ANTIICE ON". If I were to then turn the wing anti ice on (heats the leading edge of the wing, we'll just call it wing), the previous message would then be replaced with "WING/COWL ANTIICE ON". These are all green messages, green meaning good. If, with all of this icing equipment on, we were to fly into icing conditions, and the ice detectors on the sides of the aircraft detect an ice buildup, another geen advisory message pops up: "ICE". That is all that happens.
Now let's take an example of what happens if we don't have everything on. If, for example, we have the cowls on but not the wings (as would be the case when flying into an expected icing area), and the ice detectors find ice, the following things happen:
1) We get an aural "ding" in our headsets and over the speakers
2) The master caution lights start flashing brightly in our face
3) A yellow caution message appears on the EICAS that says "ICE"
4) A yellow light appears on the overhead panel next to the antiice switches.
Pretty hard to ignore, right? If we were to then turn on all of the anti ice (wings and cowls), all of the yellow lights go out, the caution message goes away, and the green "ICE" advisory message comes on.
Sounds simple? Not really. Let's briefly examine everything that just happened. This is to prevent any speculation that there was a system failure that the crew wouldn't know about.
Here is a partial list of all systems/parts that just came into play:
Ice detectors
PRSOVs (modulate bleed air from the engines)
Left and Right cowl anti ice valves
Left and Right wing anti ice valves
Wing overheat detection
Cowl overheat detection
Wing low pressure detection
Cowl low pressure detection
Bleed air duct leak detection
Wing duct leak detection
Have I made my point yet? No, that's not all of the systems/peice that just came into play. There's a lot more that just happened, al behind the scenes. The point I'm tyring to get acrossed is if just one of any of these systems and valves and sensors, etc, were to fail, the crew would know about it. I'd get a "ding" or a "ding ding ding, ANTI ICE DUCT" or "ding ding ding, BLEED AIR DUCT" aural warning plus the EICAS would show exactly what was broke. Then I'd run the QRH procedure fixing the problem, or finding out how we were then limited.
Sorry for rambling. Let's not speculate as to what happened. These pilots were professionals. They weren't just out flying around in ice willy-nilly without a care in the world not knowing what they were doing.