Yeah I don't know what "unapproved" means for their organizational structure and who would be finalizing/disseminating these sorts of notes going out, but the suggestion is that Sanders and at least his high level staff didn't authorize it. Seems perfectly plausible to me and the alternative is that they all endorsed an initiative to begin criticizing Warren, which would contravene a core principle of their campaign and inevitably blow back. At a minimum, as I said before, it's sloppy and they should tighten up if they're going to hold themselves to high standards. Missteps won't go unexploited.
This wouldnt be the first time the campaign was caught in a lie and Bernie has had to use the rogue staffers defense. To say otherwise or suggest this is unlike how they’ve behaved before would require relying on an unhealthy amount of hope.