The junior was in the wrong, so I think you'd agree that a correction is in order. Replying-all is one way to do such a correction. The alternative would be to force him to issue a retraction himself.
Or, to defer to the ever-wise Gods of the Copybook Headings:
"What's good for the goose is good for the gander".
1: https://en.wiktionary.org/wiki/what%27s_sauce_for_the_goose_...
I've been in this situation before and I've found this approach beneficial.
Some do, some think it is a sign of weakness and start behaving even more unreasonably.
But it's necessary to shame people who work the trade desks, or else they will never learn.
While there is perhaps no need to shame someone who is acting in a shameful way, I find it hard to criticize someone for doing so. Sometimes an aggressive statement deserves an aggressive response.