Seen it but never gave it a whirl. Not too sure what King Arthur would know about Chicago pizza. If you want the real deal give this thread a read.
The recipe is based on my crust formula. I am originally from Urbana, and grew up eating Papa Del's. My crust formula was derived from a mix of several from this forum, and then further modified to my taste. So what "King Arthur knows about Chicago pizza" indirectly comes largely from this forum. As an aside, the blog states that I am from Chicago - I don't think PJ Hamel, who wrote up the recipe and the blog for the site, has spent enough time in Illinois to know that C-U is not a Chicago suburb.
PJ converted it from a baker's formula to a recipe format. Some of the amounts were rounded a bit but it's basically my recipe. I make no claims as to authenticity. I live in Virginia (I lived in Tennessee when the recipe was first published on KAF) and I came up with it in order to get my stuffed pizza fix between visits to see my family in Urbana.
As for spinach, I happen to like it. When I go to Papa Del's, I generally get sausage and spinach. I use fresh spinach in my pizza, and if you read PJ's recipe and blog, you will see that she suggests leaving out or adding toppings to fit your tastes. If you don't like spinach on your pizza, don't put it in.