All US departures must be screened if departing from a Canadian airport with US Preclearance. This includes international arrivals, which you would be

If you can get your onward boarding pass issued, you can follow signs for "US Connections" as you arrive into YUL and you'll be eligible to proceed through the connections hall and present yourself in front of USCBP to continue your turn-around.

If not, just enter Canada normally, present yourself to CBSA, get to the check-in hall, and depart for the United States like normal, including the requirement of being screened at security prior to presenting yourself at USCBP.

Getting your boarding pass ahead of time is the only way to skip CBSA