User blog comment:B214/Volume 23 discussion blog/@comment-29963364-20170531001243/@comment-31137559-20170531232329

Well Rizevim was a Super Devil, far surpassing a Satan, and Apophis and Azi Dahaka were HD-class, which also far surpasses a Satan. While Grayfia and Diehauser are only Satan-class devils. Even if Rizevim was mentally exhausted, he was still one of the three strongest devils although far inferior to Sirzechs and Ajuka. Grayfia and Diehauser despite being two of the strongest devils in the Underworld would be no match against a Super Devil or Heavenly Dragon. They would simply be overwhelmed by Issei's and Vali's DxD forms due to the huge gap in power.

Putting that aside. Only being able of being HD-class for a few minutes wouldn't put you at that level. What happens when Issei and Vali have to face a god at the same level of their DxD forms? If that god is at the same level, then he can fight an extended battle with those of the same caliber while Issei and Vali could only fight a blitz and one huge attack in those forms would completely drain them and if that didn't decide the battle, then what? When Issei and Vali master those forms to an extent where they're able to fight opponents of the same caliber in an extended battle, then you can call them Super Devils.