2 things to consider:
1) Greater Dieties always maximize their rolls. You still have to roll to determine whether they critted or not, but essentially, they always hit. They always max their damage rolls, whether for attacks or spells. This is a serious crimp in diecidal plots.
2) Second, "Most dieties are 20 HD outsiders with 30 to 50 character levels as well. These additional charsacter levels beyond an effective character level of 20th follow special rules covered in more detail in the Epic Level Handbook." (Dieties and Demigods p26.) That they didn't spell it out doesn't matter.