To be honest, I don't exactly remember how I came up with 5 ounces for a cup of all-purpose flour measured out by volume. As I mentioned in the thread you referenced, the biggest problem I expected to have at the time was converting a volume of flour to weight inasmuch as there are so many different ways of measuring out flour by volume, with each method usually producing a different result. These days, I would use member November's Mass-Volume Conversion Calculator at http://tools.foodsim.com/.
But back at the time of the thread you referenced, November's calculator did not exist. If I had to guess, I would say that I perhaps assumed a method of flour measurement and went back to my kitchen and weighed the flour on my digital scale. The most common methods of flour measurement, back then and now, tend to be the Textbook method (the method the flour millers and sellers usually recommend) and the Dip or Shake method. A while back, I attempted to define those methods, and others as well, at Reply 21 at http://www.pizzamaking.com/forum/index.php/topic,6576.msg56397/topicseen.html#msg56397.
This morning, I went back to November's calculator and checked the weights for a cup of flour for each of those methods, using the Gold Medal all-purpose flour from the pull-down menu. For one cup of the GM all-purpose flour, I got 4.46 ounces using the Textbook method and 4.97 ounces using the Dip or Shake method. Since the original CPKI dough recipe also mentioned using bread flour, I checked the corresponding weights for one cup of bread flour, using the King Arthur bread flour from the pull-down menu, and got 4.56 ounces for the Textbook method and 5.07 ounces for the Dip or Shake method. Based on these numbers, it is possible that I used the Dip or Shake method of flour measurement (but more likely the Dip method) to do the conversion of the CPKI recipe.