The official word is: if he played it, it wasn't from Bungie. So Casamassina's probably not insane or lying, but it's not anything resembling a "real" Halo DS with the official stamp of Bungie approval. Apparently, the idea of Halo on the DS has been pitched numerous times, so Jarrard and O'Connor said that if Casamassina played a handheld Halo, it was probably a prototype mockup for one of those pitches. Sorry to dash any hopes, but the Bungie duo didn't sound too enthused on the idea of Halo on Nintendo's dual screens. They wouldn't rule it out, but it seems we're not the only ones who have a few reservations about FPS games on the DS. That doesn't mean there couldn't be a Halo spinoff of some sort, however!
We can only hope this puts a stop to the discussions of whether or not Halo DS ever existed, even if it doesn't really answer the question. We can, at least, say for sure that Bungie didn't make an official version, but maybe your neighbor Bob is working on one right now. Why don't you go give him a hand?