The Nascent Soul Sword Cultivation Puppet was simply not an entity he could contend with.
In his last attempt, he had gone all out, staking ninety-nine percent of his lifespan into that strike with the "Heavenly Sword" second style. He unleashed the strongest sword strike of his life. Yet, it only managed to sever one arm of the Nascent Soul Sword Cultivation Puppet. That was the strongest move he was capable of, also the strongest move of his life.
Without destruction, there cannot be construction.
With that sword strike, he was left with just one year of life.
He was gambling that this strike would slay the Nascent Soul Sword Cultivation Puppet. If he won the gamble, he could travel to Xuantian Continent and achieve a breakthrough from the Nascent Soul, thus extending his lifespan. But... he lost the gamble.
His strongest strike of a lifetime was but child's play in the eyes of the Nascent Soul Sword Cultivation Puppet.