>If a random AC employee gave you a free flight, on the other hand, you'd be entitled to it.
The company would be entirely within their rights to say 'this employee was wrong, that is not our policy, goodbye!'. This happens all the time with more minor incidents.
No idea about the US but this very same case was tested in France and some part of Germany in the late 90s when some PayTVs (Sky or Canal+, can't remember) tried to cancel multiple subscriptions offered with an extremely aggressive pricing by some of their agents. Courts concluded that the signed agreements superseded the official pricing and they had to offer the service for the entire length of the original subscription.
Where I live, "meeting of the minds" is necessary for a contract. Written or not. In this case, that meeting didn't happen. Due to the bullshit generator employed by Air Canada.
So there was no contract but a consumed flight. The court has to retroactively figure out a reasonable contract in such cases. That Air Canada couldn't just apply the reduced rate once they learned of their wrong communication marks them as incredibly petty.
That's far less likely to be true if the customer buys something based on the employee's erroneous statement. I suspect in an otherwise-identical case with a human customer service agent, the same judge would have ruled Air Canada must honor the offer.
The company would be entirely within their rights to say 'this employee was wrong, that is not our policy, goodbye!'. This happens all the time with more minor incidents.