“The reason why I asked is that there are rūpa(hadaya vatthu) in arūpa realm. So arūpa dhātu is not same the arūpa loka.”
Yes. There are rūpa (hadaya vatthu of the Brahmas) in Arupa loka.
Arūpa dhātu are rupa below the suddhāṭṭhaka stage, i.e., those are kammic energies or dhammā.
That is an important point. See “What are Rūpa? – Dhammā are Rūpa too!”
So, the sutta in question, “Dhātu Sutta (Iti 51)” states a highly condensed idea/explanation.
– All “tangible rupa” in this world can be put under rūpa dhātu.
– They all arise due to arūpa dhātu (or kamma bija or dhammā)
Stopping the arising of arūpa dhātu (or kamma bija or dhammā) will stop the arising of rūpa dhātu and will lead to nirodha dhātu or Nibbana!