How are they atheist? The only fact we get from that text is that they don't believe in an afterlife. Beliefs where one or several god figures that created the planet exist, but there's neither an afterlife nor reincarnation for the mortals, do exist.

Ajaxandriel wrote: »

I read that in a shelf, it's new chapter

https://en.uesp.net/wiki/Lore:Cries_from_Empty_Mouths

Interestingly enough, it's seems they made the Lefthanded Elves a kind of "flat-earth atheists" of Aurbis
https://tvtropes.org/pmwiki/pmwiki.php/Main/FlatEarthAtheist

I considered the possibility that the various afterlives didn’t use to exist, and consistent belief in them brought them into being, but the Yokudan afterlife specifically has been spelled out as a way to endure the kalpha cycle, and I’m pretty sure that predates the Sinestral mer. So there would definitely be afterlives in existence when the author of that book is questioning their existence.

The mer in general don’t seem to be big on afterlives: the Altmer and Dunmer have their ancestors’ spirits hanging out in their tombs, and the Bosmer’s Ouze is right in Valenwood. Presumably some of them go off to the Aedic afterlives but they don’t seem to have a racial default the way the Nords (Sovngarde), Khajiit (Sands Behind the Stars), Redguards (Far Shores), and Argonians (Hist) do.

Actually, we would also have to keep in mind several aspects surrounding the text at hand if we want to interpret it: Who has written it? When? What did the writer have in mind, and who was the text written for? That's all information that we lack.

A text doesn't have to be a factual account, it could also be written to declare superiority of one group over another or even to morally destabilize an enemy, just to name a few examples.

And then, the translator even mentions having problems translating the text and having used the Yokudan language as a base. Which could have lead to translation errors, even if the result looks about right to the translator. Whose perspective is also skewed by his personal understanding of the world - imagine there's a foreign text about a foreign cultural concept, and someone whose culture doesn't have these things tries to make sense of it - the interpretation will most probably be influenced by the translators's world view (happens very often in the real world, too, don't ask me how often I've come across people thinking that Buddha is the god of Buddhism akin to the god worship of the Western monotheistic religions, because from their own experience a religion is the worship of such a god, and they can't imagine otherwise); they might not even be able to fully understand the whole thing the way it was originally meant. Also: different cultures have different metaphors, different symbols, that might be lost if the translator isn't familar with them.

Just to remember that we shouldn't treat the new lorebook as describing facts.

Ajaxandriel wrote: »

I mean the trope (look at the link above) not literally having no gods - this we have no clue about. The matter of gods or not isn't in the text.

Then the trope has a stupid name because a-theism literally means "without god(s)"

Ajaxandriel wrote: »

There obviously is an afterlife in Aurbis (or is there?) as we witness many spirits and so on.
Yet they don't believe in it.

To quote from the new lorebook:
"While many stages of death exist, in the final after there is nothing."
These spririts (or states of existance) could be considered a stage between life and the "final nothing" of the Sinistral's religion.

Also, it seems quite obvious to me that there's a mocking tone in the narration. Like "In the end, the real end, everything will be gone - the people who believe otherwise (who happen to be our enemies) are fools".

As well as the argument for rejecting the idea of an afterlife from the point of view of the Sinistrals:
"Knowing this makes our people strong. We tell no stories for comfort, so we fight to stay in the here and now."

If one looks closer, it's actually the description of two different, "competing" methods how religion and the idea of death is used for encouraging warriors: The Yokudans fight vigorously because they think it will gain them points for the afterlife - the Sinistrals do it because they do not believe in an afterlife and want to survive in the present world of the living. From a narrative point of view, it emphasizes the differentness between the enemies. I have no clue what the intention of the writer (the one from ZOS) was for this story, of course, but I do think this is an interesting contraposition.

The choice of calling these elves Left-Handed/Sinistral is also interesting, considering what the term means in the real world. In modern media, it's often made to a "black magic" cliché - which is very simplified (and also a subjective interpretation, btw). Originally it meant a rather... how to say it? A focus on the living world, an inclusion of carnal aspects, of the physical, instead of being solely focussed on mind and spirit. And this is exactly what we also see in this lorebook - there's a focus on the here and now by the Sinistrals, in comparison of the Yokudan's focussing on the afterlife.

And furthermore, historically (already since the Middle Ages) , "right" was associated with good or correct, "left" with evil or odd. Considering the other religions of Nirn, the Sinistrals are indeed the "odd" ones.

Lets try to keep this TES Lore

As has been said, the text is not just mistranslated, the significance of some parts of it may be beyond the translator's understanding. Nevertheless, some of this fits well with other snippets I have read in TES Lore...

The Kanuryai battle sage says, "While many stages of death exist, in the final after there is nothing." This describes the psijic endeavour (aka Lorkhan's test). The word "nothing" refers to Sithis/Psjjjj i.e. God.

In the last line the Kanuryai narrator has an epiphany, "That which I see and feel is the truth." He finds that inner self which TES Lore assiduously refuses to name. Auditorial Notes, Declension 81u5 decribes it as "the essence of consciousness". I think it's what Cadwell calls, "the part of me that makes me me". A spark of streaming consciousness that is entirely subjective, and objectively nothing, which, as the battle sage said, will eventually return to its source - to nothing (Sithis/Psjjjj i.e. God).

That is not atheism, far from it.

P.S. The translator missed it, but I think the poetry is introspection, the prose is not. The poetry is on the inside