It seemed to me that he was mainly testing Snape's loyalty byt telling him Tonks and Mad-Eye were vulnerable and that there was no protection against the Dementors if they came through the forest. None of these things are true, but if Snape was on Lord Thingy's side, he'd launch an attack immediately. He couldn't tell this to Augustus Pye, because Snape would have known Augustus wasn't telling the whole truth.