Small correction: you can't board a flight to Canada. You're always welcome at a land border without a test report in hand. In which case you'll be required to spend 14 days in government quarantine.
Canadian citizens and permanent residents always have a right to enter. Presumably if you come to a land border without a negative test, you won't be trusted to self-quarantine properly and will be whisked off into a federal facility.
https://www.canada.ca/en/transport-canada/news/2020/12/pre-d...