The Captain and F/O were both quite experienced. Because he was seriously fatigued, the Captain asked the F/O to act as PF, while he acted as PM. This was contrary to company policy as they had not met before. During the briefing and approach the crew did not fully register that although the glideslope was transmitting, it should not be relied on as it was affected by snow buildups around the antenna.  

As a result the crew used ILS minima instead of localiser-only minima, and continued the approach even though the visibility and RVR were below the correct (localiser only) approach ban values. After passing the outer marker the RVR deteriorated to 2000 ft, below the value for commencing an approach but legitimate for continuing using the full ILS minima.   There was a crosswind from the right (30 - 40 degrees at 16-22kts).  

The report does not contain detailed output from the Flight Data Recorder. From the CVR it is evident that the Captain stated that he had some external cues at about 500ft.  However, it was not until 10 feet below the (incorrect) DH that he called some lights in sight, to which the F/O responded "continuing" . 

At about 80 ft the Captain had lost sight of the runway and thought a go-around should be initiated, but did not positively initiate it; at 40 ft the F/O stated he had the runway end in sight.

The aircraft touched down half way along the 6000ft runway. Braking and reverse thrust were not properly deployed and the aircraft ran off the runway at about 40kts.

The crew procedures are not detailed in the report and it appears that in this respect the crew call-outs actions were not regarded as unusual. From their comments it appears that both pilots were head-up below DH with the autopilot engaged  - the only references in the report are an auto callout of "auto-pilot" at 40 ft. 

It is very clear that the even disregarding the use of incorrect DH/RVR minima, at DH the F/O had not satisfied himself from the visual cues that the aircraft's position and rate of change of position were satisfactory - the basic Decision Height criteria. However the report makes no comment about when the autopilot was disengaged or the aircraft trajectory over the threshold.

It does however note that a go-around should have been initiated because the Captain had lost sight of the runway below DH, and also that if the correct minima had been used, a go-around would almost certainly have been flown due to lack of visual cues. 

Leaving aside the issue of whether the F/O should have been PF and intending to make the landing in marginal weather, the crew procedures did not result in a clear decision at even the incorrect low DH. The Captain's callout that some visual cues existed at around 500ft probably increased the F/O's expectation that a landing would be possible despite the conditions.  Both pilots were involved in the land / go-around decision, and the wrong option was taken.    

If a PicMA procedure had been in use, even with the F/O landing:

1) the crew would likely have had significantly more discussion of the landing on the contaminated runway;

2) there would have been a clearer decision announced at DH, probably followed by a go-around.