I also do not get it.

"Pension plans are required to measure the size of their funding obligation based on long-term bond yields, so an 80-basis-point increase in bond yields in the final quarter of 2016 led to a strong turnaround in funding"

If the pension funds just held their bond portfolio thru the last quarter then there is NO CHANGE in the yield on their holdings (same coupon,  same purchase price).  If they added large amounts (new purchases) to their bond holdings then the average yield could rise to some degree.

99% of the investing public does not understand bonds and articles like this one do not help that situation one bit