Announcing you as "No longer one of Jehovah's Witnesses" was a legal move. Otherwise, they'd still say "so-and-so has been disfellowshipped" or "has disassociated themselves".
When you're DF'ed, I was under the impression that you can still carry your "No Blood" thingy...that you're being punished until you've repented enough in the elders' eyes to be let back in. So, in my view, re-baptism wouldn't have been necessary back when they were still announcing that you were disfellowshipped because you're still recognized as a JW, just being punished. That's what I always thought.
Disassociation, however, was a different matter. When it was announced that someone DA'ed, then they were no longer viewed as a JW...and so the reasoning in the OP stands for that case.
But now that they just say "no longer one of Jehovah's Witnesses"...well, that kind of puts a new slant on things and, in that case, yeah a re-baptism should be required.
Though, who in their right mind would make that mistake twice???