Nothing to do with ISD.
PAP has an IB which regularly inspects popular online news sites, blogs, forums and social media to see if anything is objectionable. Their focus is on those who write with real names and have a good sized following because that is where the impact factor is.
Objectionable content is sent to the PAP legal team to decide if legal measures are necessary. If so, then the aggrieved party is alerted to the defamatory content and told that he has option to sue. Lawyers will be on standby. For LHL, Davinder would have been kept in the loop even before it got onto LHL's desk.
Sometimes, the minister or MP's own sycophants will spot objectionable content related to their boss, and alert their boss directly. But usually the boss will still consult the legal team on appropriate course of action because the act of suing has political ramifications so the rest of the team must be kept in the loop. If the minister or MP decides to sue, it will be because the rest have agreed that political impact is acceptable. Again, lawyers will be in the loop from the start.
If the content is outright defamatory and very severe, the minister or MP is expected to sue or he will face music from the rest of cabinet.