I've never made a pork sirloin pork roast before. In your pork sirloin recipe, did you add any liquid to your dutch oven? Did you turn it during cooking or baste it with juices? Thanks!
Pork loin can be roasted without liquid, or braised with liquid, it's your choice depending on the end result you want. The main concern if roasting dry would be to not overcook the meat as it will dry out, so if you want to do that make sure you use an oven thermometer and don't cook it above 170F at the absolute maximum. When roasting I cook to 155F and get it out as I know it will go up another 5-10 degrees after I take it out of the oven.
Wet roasting (braising) is a more forgiving method as the cooking liquid will take the place of fat in the meat itself and give both flavor and good mouth feel. The longer you braise the more tender the meat will be as well. Just make sure you cover when you cook so you don't lose the moisture.