If, when claiming two things are sufficient, you have to introduce a third thing (political structure) to qualify them, you're clearly oversimplifying it too much, leading to an erroneous conclusion.

Of course it’s a simplification, but so is E = mc². A good model strips things down to the variables that matter most for the point you’re making. GDP and population, viewed through the lens of political structure, give you a workable baseline without drowning in secondary factors.