The clue here, methinks, is likely to be found in the details: the USPS, as currently configured, is not the same as the USPO (aka the “Postal Service” for the first 195 years after its establishment).
Our friend Lake is correct about the Postal Reorganization Act of 1970 making a sea-change, and creating the “government corporation” overseen by a Board of Governors which currently obtains - and I’m fairly certain that he’s making just such a distinction when pointing out who was Postmaster General when President Johnson II’s (LBJ) ‘second administration’ began supporting the unionized government employees’ wildcat strike which ultimately led to that bill which President Nixon into law*.
*
It may also be noteworthy that the same Postmaster General was undoubtedly instrumental in the 1967 decision to rescind the long-extant RPO (“Railway Post Office”) functionalities - which was by and large undertaken under the same sorts of political pressure to stop the bleeding of money as every P.G. since then (and some, before) has been obliged to face.