The difference was that the Withering Time Technique consumed one's own lifespan to manipulate the flow of time, whereas the Nirvana Decree clearly did not require this.
The Power of Nirvana which the Nirvana Decree carried employed time as energy to corrode anything it touched; the spiritual essence of his Spirit Sword in his hand didn't weaken because it was absorbed by the Power of Nirvana, but rather because the lifespan of the Spirit Sword had been reduced.
Magical Treasures and Cultivators were somewhat similar; they also had a lifespan limit.
A Void Instrument-level Spirit Sword, if unable to absorb energy, would exhaust its spiritual essence in two thousand years, falling to the level of a common Magical Treasure.
With each clash against his opponent, the lifespan of his Spirit Sword was reduced, albeit not by much; with the opponent's Power of Nirvana, it would take at least dozens of times before his sword completely lost its spiritual essence.
