Originally Posted by TheGreatTree

While I've never gone through this process before, is it just an immigration check or actual customs check (for goods) to go from international arrivals straight into the pre clearance area?

A counter point to your "cleared as admissible to Canada" would be the CTP and TWOV Program. While these passengers are "admissible" to Canada to allow them to reach the pre-clearance facilities, they are not admissible to enter Canada proper without getting an exemption from CBSA, which may occur if a flight is cancelled.