Personal view but the moment you pass through immigration and customs away from airside you will be in Canada even if in arrivals hall and then departures hall therefore subject to the self isolation. Not sure that a return flight would be considered a flight that would come under the connecting /transit banner at a connections desk without having to exit immigration/customs and proceed to a checkin desk then back through security and US CBP at the airport
Really you need to check with your airline what is possible for the return but you should maybe expect to be subject to being questioned by CBSA what your isolation plans are. It really is not a good time to travel to Canada for any reason unless deemed essential, a soft landing not considered essential possibly by Canada or the US on the return.
As per all the other posts/answers here raise a web form to be registered in the system as having potential travel issues