It's strange if that rule is true. Why? Why should I buy return tickets to go to China? If that is the rule of Chinese government, then it applies to all other air lines. But seems airlines other than Air Canada don't care it. So I doubt the truth of such rule.
This rule is real and is set by the Canadian Custom. Air Canada is simply following the law. If you do not have a visa that is longer than 1 year, You MUST have a round-way ticket to get aboard.