I’d be willing to bet that Trump and Mad Dog are trolling the press. They both seem to like F’ing with them.
Nobody is safe. Tillerson was fired, and Nikki was let go with dignity. Mattis could be suddenly gone one day and he knows it too.
None of this is any actual indication of whether Mattis will be working for DJT next Wednesday. Often such a declaration precedes a termination by days or weeks. Often it is accurate. It means actually nothing.