Only defined contribution pension plans are required to be fully funded...because of the nature of the pension (the plan guarantees the amount input but not the benefits available when the worker retires). A defined benefit plan can be fully funded, but ERISA does not require this--indeed, few defined-benefit pension plans, public or private, are fully funded. Moreover, fully funded in this context only refers to the funding of vested benefits. The USPS is required to pre-fund all benefits for current and past employees, even those benefits which have not yet vested.
Also, please note that in other posts you are conflating 401-k contributions with pension contributions. They are not even remotely the same thing.