Where exactly does it say that?
http://www.aircanada.com/en/travelin...r%20Canada.pdf
Page 41. Maximum baggage allowance is according to the table on that page.
The carriers in the table include AC only. There is nothing listed for other carriers.
I am not a lawyer and don't claim to have the definitive interpretation. I surmised that when the tariff refers to "the carrier" and "other carriers" that when there is a table of baggage maxima only for AC, that "other carriers" might have different rules.