The president's press secretary is completely different than an anonymous spokesperson supposedly being quoted by some random reporter. Let me point out the obvious --- when it's an actual spokesperson for a federal agency YOU GET A NAME ATTACHED to the message - not just some anonymous source. The person talking to the press is identified for verification of status. In the article you link we have nothing more than an anonymous source - not really a spokesperson. We have no idea who that person is or their actual title/authority. Clearly this is bush-league journalism at best and obviously should make you question the message.
So once again - Mnuchin DID NOT say what you claimed. Some anonymous person did. You can choose to believe it - that's your choice. But you can't say Mnuchin said it - because he didn't.
When a spokesperson speaks, they do so under the direction of who they represent. That's the entire purpose of a spokesperson. The spokesperson for the president speaks almost on a daily basis. Guess why?