It does require some common sense. I recall Kim Bruning noting with amusement that a comment from Patrick Nielsen-Hayden on an AFD discussion on a science fiction author probably counted as a reliable source in that field ;-)
If it was verifiably him, then it probably was. Far from an ideal source, though. IAR applies to the main namespace just as it applies everywhere else - the including an AFD discussion as a source makes the encyclopaedia better, go for it.