So this rather dozy crew turn off the switch without adding power
No, The switch was on, but they did not bug or fly the associated higher speeds. If the pilot had reacted to the shaker by simply reaching over to select the INCR REF switch off, that would have reset the stall warning angle of attack and the shaker would have quit and the crew would likely have been able to continue the approach and land safely without further incident.
No it was not erroneous - the system operated exactly as it was supposed to.
The stall warning/shaker went off at a lower preset angle of attack meant to represent a contaminated wing, but the NTSB has determined that icing contamination was not a factor. Therefore, in the absence of significant ice contamination the shaker/warning was erroneous. The
equipment worked as it was supposed to, but the necessary crew participation to make the
system work was absent. No system is perfect and the INCR REF switch portion of the ice protection system on the Q400 is perhaps one of the least perfect. Its definitely not of the set and forget variety that this crew seemed to believe it was.