It has been stated many times that the angel and demon tablets can only be read by a Prophet of the Lord, no one else. Angels and demons cannot read them, not even extremely powerful beings such as Knights of Hell, Princes of Hell, Archangels or even Jack can read them. Yet somehow Rowena is able to read the demon tablet, she may now be the most powerful witch to ever live, but her power and ability does not surpass that of an Archangel.