Just out of random curiosity, on the press release it says the following:
Air travellers who qualify as fully vaccinated and who are selected for mandatory random testing, as well as air travellers who do not qualify as fully vaccinated, will receive an email notification within 15 minutes of completing their customs declaration.
Does that mean if you complete the Advance CBSA Declaration on the ArriveCAN app in the 72 hours before you travel, you'd find out before you even start travelling whether or not you'd been selected, or are you possibly selected at customs in the airport?