Because it doesn't matter when the belts were made, as long as they were made for the military, for that firearm.
"(f) is of the “belt” type consisting of a fabric or metal belt, is not a reproduction and was originally designed or manufactured for the purpose of feeding cartridges into a automatic firearm of a type that was in existence before 1945."
CC Regulations Prescribing Certain Firearms and other Weapons, Components and Parts of Weapons, Accessories, Cartridge Magazines, Ammunition and Projectiles as Prohibited or Restricted
Part 4 3. (1.) (f)
http://laws-lois.justice.gc.ca/eng/r.../fulltext.html