Good idea. Though I think the fact that they trusted the departing person, and the departing person delivered -- if it plays out that way -- is much more positive message than effectively implying that the company didn't trust the person.
The company letting a manager relay a message, with any censoring, is certainly better than the person having no way to get their contact info to people, and they might also say something nice for morale.
You are right that the other way sends a stronger signal.
My suggestion was meant as something that's feasible even for a company that already got burned by vengeful leavers; and also something that an individual manager has an easier chance of pulling off, without having to change all of corporate policy.
The company letting a manager relay a message, with any censoring, is certainly better than the person having no way to get their contact info to people, and they might also say something nice for morale.