As it happens, in between first reading about ZFS dedup, and finally trying it out, I seem to have forgotten that the verify option existed. I just did "set dedup=on" - beyond that everything was whatever defaults you get on OpenIndiana build 148.
Were I to ponder that matter to any great depth, I suspect I'd find it rather difficult to get a handle on how concerned I ought to be about hash collision. Perhaps that's part of the answer to your puzzlement.
Because they believe in statistically low probability of that happening.
What they don't accept is that it might hit them in the worst time possible, that a backup of such information will also suffer the same problem, that after an upgrade an important part of the new kernel might have the same hash as the beginning of Morissette's "Ironic" mp3 you already store, etc.
Were I to ponder that matter to any great depth, I suspect I'd find it rather difficult to get a handle on how concerned I ought to be about hash collision. Perhaps that's part of the answer to your puzzlement.