The authors speculate that this may be the result of Hong Kong officials trying to demonstrate their loyalty to Beijing, where the use of “so-called” as a general term of abuse in official statements is well established. Ignorance is a possible alternative explanation.
The meaning of “so-called” is well established. It indicates doubt about the validity of the noun which is coming next. For example, someone who had doubts about the independence of the Independent Police Complaints Council might prefer to call it the “so-called Independent Police Complaints Council”.