In mathematics, an "if and only if" statement is defined as being true whenever its arguments are both true, or both false. "Snow is white" and "that's what Eliezer Yudkowsky wants to believe" are both true, so the statement is true.
Statements containing "if" often (usually?) have an implied "for all" in them, though. The implication here is something like "For all possible values of what-Eliezer-Yudkowsky-wants-to-believe, snow is white if and only if that's what Eliezer Yudkowsky wants to believe."
Hm. Yeah, that's how I read it. I'd say it this way, when I see an "if and only if", I see a statement about the whole truth table, not just the particular values of p and q that happen to hold. This is a mistake?
If you know more Eliezer Yudkowsky facts, post them in the comments.