To be clear, I'm not necessarily saying that Don Cossack has been "taken out of his comfort zone" in these races.
It's more about his style of running. When he is duffing-up G2 and G3 horses, as he has done in the JN Wines and Kinloch Brae this season, he can travel on the bridle throughout the race, and is essentially unhurried as he approaches his fences. But put him in against G1 horses, and he doesn't travel nearly so well, and he can make the kind of mistake he would never ordinarily make against more moderate opposition.
This theory aside, I think he is thoroughly genuine; he just races lazily by nature, and this is magnified to an extent, when better horses are going a more thorough gallop. This, imo, is a different thing than him being "taken out of his comfort zone". He always gives his running, and I haven't seen a race over fences yet, where he has looked out-classed.