Do Xyz Materials ever “leave the field”? Once they stop being monsters and start being Xyz Materials, they’re no longer considered “cards on the field”, but they’re still sitting there right in front of you. When they’re detached, they go to the Graveyard which is something that most people consider “leaving the field”. So it seems, initially, that the card would leave the field twice. But then, you have to reconcile all of this with cards like Reborn Tengu that cannot activate their “leaves the field” effect when they are returned from the field to the Deck even though they clearly had to leave to get there. That’s why we included the section on “leaves the field”.
This section specifically defines what will and will not trigger a “leaves the field” effect. A card on the field that is sent to the Graveyard, banished, or returned to the hand or Extra Deck can activate its “leaves the field” effect. A card that is shuffled into the Main Deck or becomes Xyz Material cannot. I think you’ll agree that it’s much easier to say “If this card leaves the field” rather than “If this card on the field is sent to the Graveyard, banished, or returned to the hand or Extra Deck” and gives us more room to tell you what the card actually does.
Basically what Konami is saying is that when you detach monsters like Reborn Tengu or Sangan, their effect do not activate. If a monster is used for Xyz summoning and later “detached” from the Xyz monster it isn’t the same as being a regular monster. A Xyz material is basically a counter for the Xyz monster and technically is not on the field.