They say to cook until 190F because that's the temperature at which the stuff that actually makes your slow-roasted pork moist, the collagen, fat, etc. is breaking down and coating the meat. Less than that and you'll have all those bits still intact in your shoulder, which you don't want.
ATK explains this in their footnote on the recipe:
Just like in a pot roast, cooking the pork low and slow (325 degrees for 5 to 6 hours) pushes the meat well beyond its “done” mark into the 190-degree range, encouraging intramuscular fat to melt, collagen to break down and tenderize the meat, and the fat cap to render and crisp.
ATK's foot and header notes have taught me a lot over the years and I highly recommend them.