First, it's a 6th level Wizard spell. Now, who, of the four characters here, do you think has enough ranks in Knowledge (Religion) to know that the spell is a higher level for clerics using it from the Destruction domain than it is for Vaarsuvius, their teammate, who uses it all the time? (Answer: None of them.)
And second, don't tell me I made a mistake in my own comic. I could not give one good goddamn what the rules say about anything. There are no mistakes, because there are no rules. NONE. No, not even that one. Further, I do not care what you think is a "reasonable" assumption for them to have made. This is the assumption they DID make. Period. End of discussion.