I'd say it's a loophole.
His post is on topic because he gives his opinion to the @OP's post and it's constructive as long as his post has a weight, right?
I sort of agree with you but not no the constructive part one as he is commenting on something already proven false.
Reporting it to mods might be a help? Since it is proven false, and probably considered a spam. Or just message the manager to take a look on what's the participant's post behaviour.