Since the protagonist doesn't have his memory, he doesn't know for sure if this is true (presumably this is due to brain damage from Bennie's shot to his head). One of two things seems true though - either the courier knew what was in the package and what it would do and did it anyway out of a sense of duty to the NCR, or he was just a mailman delivering a package.
Or there are other possibilities -- that the ED-E was not intended to cause that destruction -- after all lots of NCR soldiers were caught in the disaster. Perhaps the ED-E was sent to help the NCR get into the silos. Perhaps it was Ulysses who brought the ED-E unknowingly and has had a psychotic break (and Ulysses is seriously crazy right). Since there seem to be nuclear missiles left in hopeville, maybe there were just earthquakes which triggered some nukes and it wasn't ED-E at all.
By his own admission, Ulysses was responsible for the whole Honest Hearts scenario. He armed and aimed the White Legs at New Canaan and Zion. He is clearly guilty of an act which would certainly be just as "bad" as bring ED-E to Hopeville, if it did set off the nukes intentionally and the courier knew it would. So is the Courier just as "bad" as Ulysses, or is Ulysses blaming the mailman for delivering a bomb in the mail?