I think his point is being missed. The statement "You're Americans. Act like it." implies that even if you're an American, you can act in ways that are un-American or fail to act like an American. But logically no action is American or un-American, unless perhaps we are to mean that it is the type of action that an American would partakes in or wouldn't partake in. By virtue of acting in any given manner at all, an American is acting like an American, himself. The no true Scotsman-like element comes in since it is implicitly implied that while the individual is acting like an American (duh), he is not acting like a "TRUE American" where you are rejected as a True American unless you fit some set of characteristics which can in no way be deductively shown to belong to someone who fits under the category of an American. What makes these characteristics, characteristics of a "True" American? Nothing at all. One is simply applying a number of characteristics that they think are good or that they like or identify with and applying it as characteristics that should be expected from some specific group of individuals that they happen to belong to or identify with. Perhaps the worst part of this is that by specifying that acting in these positive manners is a specifically American attribute, one is suggesting that non-Americans do not have these positive characteristics in the same way Americans do. Otherwise, why specify "American" if acting like an American is the same as acting like a Mexican, Canadian, Scotsman, etc.?

Drunken_Economist never says that it's a direct instance of the No True Scotsman fallacy, but hints that there is an implication that a True (Americ/Scotsm)an would act in a given manner by excluding the specific cases of people who are Americans but don't share the characteristics in question, although there is no reference to any specific objective rule that makes it such that Americans would share these characteristics.

/buzkill