The article doesn’t say whether they were Russian, Chicom, or Korcom fighters (unless I missed it).
My guess is that it must have happened on the other side of the Yalu or something like that, hence the secrecy.
If that is the case, I don’t see a problem with him disclosing it. I agree it is problematic for individuals deciding what classified material they will release or not, but I can’t see where this makes much difference.
I did see where they mentioned “russian” fighters, but it was unclear to me if it was simply sloppy writing by journalists about military matters that we have become accustomed to.