Mitra and Varuna devas are adityas,which is considered different group from Rudras,While Rig Veda book 5 hymn 70 calls Them Rudras.Is it there a specific Reason or they have compared them both to Rudras or the verse is addressed To Rudras?
This is a problem of translation. You are right in saying that Varuna and Mitra belongs to Adtiya gan as described in various hymns. Rudra gan is a different group, only Maruts are designated as Rudra. In this particular Sukta (verse) RV 5.70 only two hymns (2) and (3) mention Rudra that too in conjunction with Mitra-Varuna. In the hymn (2) -"vayam te Rudra shyam" may mean " your (Mitra-Varun's) relation/connection with Rudra be hilarious. Similarly, the hymn (3) could be translated as - "And protect as good protector (uta trayae yatha su trata), with advent of our Rudra fighters (Patam no Rudra payu bhir). Move away robbers with their bodies (turyam dasyun tanu bhir). I have seen many translations commit the same mistake as pointed out by you. We have to be carefull. Any way, many words of RigVeda are obscure to us .
The aspect of Rudra is terror, so in this hymn the poet wanted Mitra and Varun to fill the heart of his enemies with terror and lay his vengeance upon his enemies like the Rudra would do.
- Since now there is help from you two, even in (its full) quantity and breadth, I have won the favor of you two, o Varuṇa and Mitra.
- We would attain you two together, o you without deception, and your refreshment for our nurture. We would be such ones, o you two Rudras.
- Protect us, Rudras, by your protections, and rescue us, since you are good rescuers. We in our own persons would overcome the Dasyus.
- O you of undeceived will, may we in our own persons not endure (becoming) the specter of a nobody, neither in our remains nor in our lineage.