I think you are misunderstanding "truth" value. Since Mapleleafs89 says "The truth value is 1 (true)", he is clearly assigning "1" to any true statement and some other number (probably 0) to any false statement. The statement is true, and so its "truth value" is 1, because this there is such a real number. The fact that the value happens to be -1 doesn't affect the "truth value".