Caught lots of lake trout on flies, mostly in the spring after ice off on lakes. Never had one take a dry but then I never fished dries, most came to streamer or clousers off a type six or seven. Even used some T-14 and 17 .
Lake trout will take dry flies when they are near surface and food is abundant. I caught them in Lake Superior during a fall of tent catapiller moths, and have caught them in the boundary waters during mayfly hatches as well. The water has to be cold and the fish shallow for this to happen. I am sure it is more common in the far north.