You are not using your artifact activation to reactivate the same artifact, you are using a special ability. Legal play.
The main thing you aren't supposed to be able to do would be something like have Covenant with Death, deactivate it, play a bunch of territory-class enhancements, and then straight up reactivate it. However, Urim and Thummim has a special ability that allows you to activate an artifact, which can be any artifact you have available. The same is true of The Meal in Emmaus, which allows you to activate a different artifact.
U+T is just silly, yo.
