Wouldn't that mean that Sahn-Uzal would've found the afterlife he was expecting, since it was a fundamental part of his culture?
In theory, yes. But there is something more at work, which we are not entirely sure about. Something may have destroyed it, the original state of Mitna served some other purpose, his soul may have been compromised in some way during his life or he simply wasn't worthy. Perhaps, for all his posturing, he never was quite ready to let his days of conquering go.
Also, we don't even know if the death realm is part of the spirit realm.
We do, from outside sources to the written lore, I know some people aren't big fans of more obscure comments by those involved, here and here for example. Or, loosely related, from Morde's writer:
As for Morde and Kindred - Different cultures of Runeterra have different understandings and relationships to death. Kindred is interesting as the myth of Kindred takes many forms across many of Runeterra's cultures. Given Sahn-Uzal (Mordekaiser when he was human)'s faith in the Hall of Bones, I think it's safe to say his culture did not have a Kindred myth. As for what he would make of Kindred were he to meet him now? That would indeed be an interesting story that I hope we get to tell some day...
Or this one:
As for Illaoi - it would indeed be interesting to see what would happen if she and Mordekaiser met / clashed. ;) The afterlife that Mordekaiser saw (and then the one he built) are what he experienced. Other cultures and other peoples on Runeterra may have very different relationships to death.
Or this one:
Different Runeterran cultures have very different relationships to Death. Seeing them clash would be pretty sweet!
(...)
Personally as a storyteller, I like having a Runeterra that still has a mysterious relationship with death, even as magic and powers allow some individuals to transcend or cross over. Did Mordekaiser end up in an empty place because that is where all souls go? Or was he deemed unworthy to access the true Hall of Bones? Death is frightening to us in real life because it is unknown. Honoring that in world seems resonant, thematically interesting, and poetically powerful. :-)
The Spirit Realm is the realm of thoughts and tales, and emotions and sensations, but it is also the realm of life and souls.