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Actually, that statement indicates the defendant knows the guilty party. If this were the case he could make a statement regarding a known truth (the sky being blue) and identify himself as Honestant or Swindlecant, and the guilty person. i.e. - "The sky is striped yellow and orange, and Joe Swindlecant didn't do it!" or "The sky is blue right now, and Joe Swindlecant is the guilty one!"
He should have said "If I am a Swindlecant I did it!". This would not only exhonorate him, but also not indicate he knew who the real guilty party was. (If he is a Swindlecant, then he can't have done it or else the statement would be true, and if he is an Honestant he couldn't have done it or the statement would be false.)
no, this statement would not in fact exonerate him... if he is a swindlecant, then he did not do it... but if he is an honestant, then the statement is meaningless to his case. this statement is if/then... not if-and-only-if. therefore, this statement does not nullify the possibility that an honestant could have done it. he would be no worse off saying "i did it."
he could have, however, said "if i am a swindlecant i did it, but otherwise i did not," or perhaps "i did this if and only if i am a swindlecant." for a swindlecant this would translate to "i didn't do this if and only if i'm a swindlecant," and for an ever-honest honestant, it would simply mean he hadn't done it.