Two systems, not one
Canadian law governs what may be supplied and for what purpose. That is the system covered in are peptides legal in Canada, and it is federal, administered under the Food and Drugs Act and enforced by Health Canada.
Anti-doping is separate and contractual. Athletes in tested sport are bound, through their sport's rules, to the World Anti-Doping Agency Prohibited List,[1] which in Canada is administered by the Canadian Centre for Ethics in Sport.[2] Those rules apply regardless of whether a substance is lawful to buy, lawful to possess, or approved as a medicine anywhere.
The practical consequence is that "it is legal to purchase" answers a question no anti-doping process ever asks.