Mattis is correct and is stating a fact.
Clearances are indeed only given and kept on a need to know basis.
Well said.
Whether former military or not, when your need to know ends your security clearance should end. It should always be based on the needs of the government and this notion of hanging on to them after separation, unless they are definitively useful to the government at a contractors employment is dangerous. Tens of thousands of clearances out there with individuals who have no more need for them.