If there is such a thing as an (ironclad agreement) the Central States withdraw language about UPS being required to pay the difference if and when there is a reduction or default has to be one. Think about it, if there was a loophole on this one they would of used it from day one and not have to wait 15 years later. As you remember they tried to get out of the "97" contract language of creating those 10,000 new inside positions once the contract was ratified, the International won it in arbitration two years later.
I believe that they have given up and are now waiting it out. In time there will be no UPS retirees with any vested time in the CSPF, all of it will eventually be under the IBT/UPS Pension trust. By the next contract and every year afterwards UPS liability goes down as the retirees pass on. With them eliminating the annual funding for their management pensions in 2023 that in itself will free up about 2 billion dollars to cover any associated costs for the possible default of the Central, which will happen short of a miracle.
I believe that they made (14 percent interest) on their three major controlled Pension funds last year. If the ecomony continues to grow at the rate it is going now they will be in hog heaven for the next couple of years. If their pension trusts are right now over 100 percent vested that means that their annual contributions would be significantly less each year just from their investment interest.