I think the issue here is the factoid (if it is indeed fact) is coming in a second hand nature to the public. Yes, DB did say she failed, but she said LE told her she failed. I guess in a court of law that would be hearsay. I'm not sure if DB was lying that LE told her, LE was lying when they told her, or it's the truth.
Is LE under obligation to tell her the truth about a poly result?
No they can lie during interrogations. I am not sure whats fact with the poly but I don't think LE is confirming anything to the attorney so his statement was to the fact the the host said LE and FBI stated DB failed a ploy and the cadaver dog hit. LE has never stated anything about the poly, DB herself said she failed. I hope that made sense lol its confusing.