I know it is a bit motivated reasoning from me because I don’t want Mattis to be guilty, but here:
Maybe the respectability of Bonanni and Foege also worked on Mattis? He knew that he couldn’t understand the science anyway, but since there were two scientists on the board who could, then Mattis figured the science behind the machine is solid.
From there he went with his knowledge of the military - if the machine works then the military could sure use it.
Mattis, like most of the board, was there simply for the big money he was being paid to lend the use of his name for the credibility of the fraudulent company. Others included former secretary of state George Shultz (who disowned his own grandson who was one of the main whistleblower employees that brought down theranos), Henry Kissinger, and former U.S. Senators Sam Nunn and Bill Frist, amongst others. All of these were old codgers who still had Washington name recognition and were simply selling their former fame for a price ...