You meant by flight, too?
Yes, the travel document is actually more for the airline than immigration. If you can enter without a travel document in a car, why not at the airport? Is it not the same kind of immigration officer standing in both places?
The airline or other commercial carrier you travel with (bus, train, ferry) is however responsible for their passangers so if you are not visa exempt to Canada, they want to see a visa or travel document to make sure you are allowed to enter Canada. Otherwise they may have to pay for your passage back.