A difficult question but am guessing if someone shows any symptoms then they would not be allowed to board a connecting flight but have to isolate at the first POE. Still does not exempt someone from having to mandatory isolate at their final destination and suspect although do not know that CBSA at first POE will be looking for an address to be provided that they can record in case compliance needs to be checked.
The challenging part though still remains no public transport use or stopping off before reaching isolation although the official publication seems to contradict this which makes little sense if someone turns out to be infected but not showing physical symptoms on arrival.