As Erin has already pointed out, Xixu Arath and Savathun were able to be resurrected because they were killed in Oryx’s throne world, not their own. At the start of XXVI: star by star by star, the card which describes him killing his sisters, it says that they are “in the throne-world of King Auryx”.
That Oryx’s sisters were not killed in their own throne worlds is confirmed in XXV: This Love Is War, where Xivu Arath says:
Once, I permitted King Oryx to kill me so that he could gain the sword logic and overcome Akka, our god. This left me trapped deep in my throne.
The true deaths that are described in XXVI do not seem to be the same as permanent deaths. A true death is a death that occurs within the ascendant realm (or “sword world” as it is called in the card) but not within the throne world of the Hive that has been killed. So they are trapped within their own throne world until they can be resurrected in some way, such as by carrying out acts that describe their nature.
In contrast, a permanent death is when a Hive is killed within its own throne world, so they cannot be resurrected. 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.
Oryx confirms that if he is killed in his own throne world, then he will have been permanently killed. 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.
As Hipnuts said, we killed Oryx in his own throne world in the King’s Fall raid. Since Oryx was killed in his own throne world, he cannot be resurrected as Xivu Arath and Savathun were.
It is worth noting that it can be said that Oryx killed his sisters in both his own throne world and the ascendant realm or sworld world since his throne world is located within the ascendant realm, as are all the other throne worlds. Toland describes the ascendant realm, or what he calls the Overworld, in Ghost Fragment: The Hellmouth:
Now I fly between green-black suns in the labyrinth beyond Crota’s god-star. This is the Overworld, the Sea of Screams, where the throne-universes of the great Hive fester in eternal majesty. I move among them. I map the shapes and connections of this world.
However, saying that Xivu Arath and Savathun were killed within the sword world does not mean that they were killed in their own throne worlds. As mentioned previously, XVII: The Weakness Verse shows that Oryx created his own throne world before he killed his sisters as described in XXVI: star by star by star. XXI: an incision confirms that all three siblings created their own throne worlds prior to the events described in XVII:
SAVATHÛN said, Auryx my brother and king, I have studied the wounds cut by the Worm our God. Also I have studied the manner of your death and return. These two things are the same, for they are predicated on death and the passage through cut spaces. Let us practice the sword logic until we are sharp. We may then cut our own wounds and step through.
But XIVU ARATH said, sister, I am already sharp, look, my sword cuts into another space. And she cut her way between moons through green fire and joyous screams.
Three kingdoms grew swollen in the sword space. They were the gaze and glory of AURYX, the cunning and knowledge of SAVATHÛN, the triumph and brawn of XIVU ARATH. These kingdoms were created from the minds and worms of our lords. They were coterminous with all spaces consecrated by our Hive. Through these spaces passed speech and food, and all the moons were bound close.
Sayeth AURYX, this is where I went when I died. Let us establish our thrones here. For I am Auryx the First Navigator and I shall chart death. And my throne shall be carved of osmium.
So when it says in XVII that they were in “the throne-world of King Auryx”, this is referring specifically to Oryx’s (or Auryx at this point) throne world, not just the sword world or ascendant realm in general.