User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-83.30.155.52-20180502153507/@comment-28884330-20180503164844

Rias didn't specify which Maou she was talking about, and her words could have just been exaggeration for the sake of flattery.

That, and fearing a person does not account only for the threat to one's own life. Sirzechs and Ajuka could have perfectly feared Strada without him representing a threat to themselves, but because he can kill other Devils who traveled to the human world, including their fellow non-Super Devil Maou.