It is not a well known term, confirmed by your need to define it. Obscure slang should not be used in sincere formal criticism of an individual's behaviour.

Additionally, the post is attempting to minimally justify the banning while assuming good-faith. It is not trying to denounce and condemn John De Goes.

Because sealioning implies malicious intent, i.e., that the guilty party's main intention in engaging in this behavior is to cause strife.

John is certainly engaging in similar behavior, but it does seem like his main problem is that he's overly dramatic and full of himself.

because the authors appear to be trying (and succeeding) at striking a more mature tone.