Ryuk was absolutely and completely right when he talked about Isha ending up saving his life.
At that last second, when that column of lava had spilled on him, Ryuk's transformation, Nao-Umbral Reaper, was already at its end.
If the transformation had had to endure that column of lava, then it would have been torn through, and Ryuk would have returned to his normal form, where he would have been burned to death by the column of flames.
Even if he somehow managed to survive getting reduced to ashes, he would most likely have been fully spent and knocked out, and Ignatius still had some little flame left to stand to his feet and be declared the winner of the match.
But when Isha came out and blocked all of the column for him, Ryuk had been able to stand unfazed by the attack, and with Isha's appearance, he was finally able to activate "King's Aura," the ultimate card that made him win the match without needing to fight anymore.
