Its not really about whether Canadian immigration officials said the passport was read-able. The passenger is, after all, going outside Canada. Hong Kong officials may not take so kindly to seeing a washed passport or damaged passport. They don't usually take the time to contact Canadian immigration officials when making a decision. They can simply reject the passport and send the customer home. Unfortunately, leaving the decision up to the customs officer in Hong Kong can be a costly decision for the airline.
In London, for example, there has been a crackdown on washed/damaged passports in the last year. Passengers with such documents are sent home and the carrier is fined thousands. If a passenger is lucky, they can get a customs officer who is willing to let the document go, but there is a risk to the airline when they decide to take that chance.
Coming home with a Canadian passport that is damaged or washed is really not an issue.