This may be an AC problem rather than a UA problem. Since the itin touches the US, the baggage policy of the first flights marketing carrier applies to the entire trip. Need to read the UA COC to see if their policy applies, or if they defer to the policy of the operating airline. If the former, AC was wrong; if the latter, UA was wrong.

edit: nevermind, I thought the first flight was UA marketed, this was some later AC flight, disregard