It seems the illegalities in the Han Dong-hoon transcript case are summarized like this

Parliamentary immunity applies only to statements made in the course of official duties within the National Assembly premises.
Posted on personal SNS → Since SNS is not the National Assembly, parliamentary immunity does not apply.
Violation of the Personal Information Protection Act → They brought up the fact that two recordings were hidden away.
Circumstances of obtaining the transcript: "Received a tip" → If the people capable of providing tips are 'Kim Seung-won', 'Mr. Yang', or 'a third party', and if anyone else provided the tip, it is all illegal (Violation of the duty to prohibit the disclosure of official secrets under Article 127 of the Criminal Act).
There is a Supreme Court precedent that punished a similar case.
2009Do1444
Summarized after watching lawyer Kim Kyung-ho's YouTube broadcast.