An offer of immunity to the chief test pilot is in the public interest. Nobody asks the lawyers to spin it, he gets immunity so all the weaknesses in the plane get located and fixed.
Just a guess- all those named in the Seattle Times article have already provided testimony to the authorities or their names would not be revealed.
You can be absolutely sure that Boeing is carefully monitoring all employees possibly involved not only on company computers and just barely within legal limits.
Months ago, the Seattle Times revealed a grand jury investigation had started.