The change in the ambiant temperature of the glass and the warmth of your breath causes external condensation on the lenses.... But when a manufacturer speaks of fogproof, that is referring to the internal glass... The glass inside the scope will not fog due to temperature change or condensation because the scope is nitrogen gas , or Argon gas , purged and sealed with multiple O-rings and seals at any place where the scope has a joint or connection to the outside.

So every scope can fog on the outside glass due to your breath, but that has nothing to do with claims of being fogproof.

MET