When people say "directly causes obesity", they don't mean it's a magic spell that reverses thermodynamics. Something that stops a large amount of calories from being burned and turns them into fat reserves deserves the title of "directly causes obesity".
So why aren't people in hot countries all obese?
Why would they be? Human metabolism evolved for hot climate after all. So, the body can regulate itself. Unless it's infected by a virus, which alters this regulation. And, unlike you imply, living in a hot country does not shift your energy usage towards fat creation like the virus does.
In the same way that a lit match tossed into a building doesn't "directly cause a fire" because it's actually a series of chemical reactions that cause the fire, I suppose that's correct.