User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-3976024-20150913082216/@comment-24834479-20150914020959

I told her what you said, and she said "That was back before they changed it so that holy fire wouldn't completely kill an archangel (for the plot of Swan Song). Perhaps Castiel assumed that holy fire had the same affect on archangels that it had on other Angels, and the archangels either didn't know its effect on them, or didn't want to get a new vessel."