That's the key though, whether a "reasonable person" (i/e someone who hasn't had legal training) would know that retweeting it was potentially libellous.
In those cases (i/e people with very few followers) he'd have been better off not even mentioning them in the first place, for the reasons you mention in the rest of the post, because it draws attention to him. Saying he was going to sue 15,000 people got much attention than if he'd gone after one or two specific people who started it or were well known.
Before that reasonable person gets as far as legal repercussions should they not stop and think whether there is any evidence , and then is it morally right to spread or repeat ? you dont need legal training for that.