I think one would really have to know the case and its facts before one could comment on exactly what’s being done and the legal justification for it.
Generally, however, the 5th Amendment applies mainly to: custodial interrogations where the government reasonably expects that the answers might be incriminating (typically post-arrest interrogations); and testimony under oath. The 5th was meant to eliminate forced testimony, and at the time it was written testimony under torture was the main concern. Later-charged suspects who said incriminating things before an investigation even began wouldn’t seem to be protected under the 5th, since nobody was forcing them to make the statements and the government was not expecting that person to say something incriminating.
Also note that the 5th Amendment right is a right that has to be asserted. If you voluntarily make incriminating statements without a lawyer (after being read your Miranda if you are in custody, or utter them under oath even without being Mirandized), those statements can be used against you.
If I tell you that I’m going to commit a crime, and you record this conversation and go to the police, I don’t have any 5th amendment protection against that evidence. But the case you’re describing has to be substantially different than this if it is newsworthy in any way.
