While it is true that Canadian citizens do not necessarily need a passport to re-enter Canada, separate requirements exist for exiting the U.S.
Canadian visitors leaving the U.S. by air must present a valid passport or Nexus Card before they are allowed to board an airplane for return to Canada. Canadians residing in the U.S. may depart the U.S. for Canada on a U.S. Permanent Resident card in lieu of a passport or Nexus Card. This card is accepted for exit from the U.S. as well as entry into Canada (but not for overseas travel).