"I still see plenty of code that does have a variable to indicate what's inside, perhaps an enum {hot, cold} or worse a bool. Yuck -- you should simply store the thing and if you need to know its state you look at it."

Now I'm confused... How are those two things different? The variable indicating what's inside is going to be coming from the actual state itself, no?

It sounds like perhaps you're talking about some case where a developer has somehow copied a state and referred to the copy somewhere else. Or are you talking about a computed variable of some part of the state being a code smell?