An undetected incorrect instrument flight path angle was NOT the cause of the accident, despite the implications of the TSB report.
On any instrument approach the exact position of the aircraft relative to the desired (perfect) flight path will vary. Depending on the degree of precision it could be closer or further away from it.
Cat 3 = very close indeed, Cat 1 precision fairly close, non-precision LOC further away, non precision NDB further still. Minimum instrument flight height (DH/MDA) naturally goes up as accuracy decreases. A Continuous Descent Final Approach NPA as used by the AC crew (whether or not using the autopilot/FPA mode etc.) drastically reduces the position scatter compared to "dive and drive, but use the same MDA.
This accident occurred when the crew descended below that minimum safe height (MDA) without having completed a visual confirmation that the aircraft's actual position and velocity (rate of change of position) showing that they were NOT going in short of the runway.
Such a descent is prohibited under ICAO Annex 6 but IS allowed under Canadian rules because Canada has a much less stringent definition of required visual reference, as already described
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When the very idea of DH/MDA was being introduced into ICAO by the FAA, correspondence with the US NTSB made it clear that "We see nothing in the definition of Decision Height which permits the decision to be made after the aircraft has descended below the prescribed height while the Captain attempts to locate the runway." But that is exactly what the Air Canada crew were doing, quite legitimately, under CARs.
For
full explanation see this