XXVI is told from the perspective of the Hive gathered in Aury’s throne world. It’s possible that what they call “true deaths” are not the same as permanent deaths. I’m not sure why “true deaths” would have a different meaning to “permanent deaths”. Nevertheless, there are other parts of the Books of Sorrow that make it clear that powerful Hive such as Oryx and his sisters can only be permanently killed if they are killed in their own throne worlds.
In XVII, The Weakness Verse, it says:
"Your body is gone, but you have endured. Safe in the cyst universe created by your own might - your throne world.
“From this day forward, Auryx, you and your sisters will each survive death - so long as you aren’t killed in your own throne.”
In L, Wormfood, Oryx says:
"I have died many times but these deaths were only temporary.
“If my echoes are killed, and I am killed in the material world, then I will be driven back to my throne the Dreadnaught. If my Court and my throne can be beaten, if I am confronted in my throne, if I am defeated there, then I will die. My work will end.”
Since Oryx killed Savathun and Xivu Arath in his throne world, not theirs, they were not permanent deaths. Instead they were trapped in their own throne worlds, as Xivu Arath describes. That’s why they could be resurrected by Oryx carrying out acts that described their natures.