This is my first real post here - so if its somewhere else thought of or such please go gently on me!
This came about because of the differences of the Shin compared to most cultures, the oddity of the location of Shinovar, the honorblades being there, amongst others.
It starts with the fact that of all the Herald's only Talenal died in the desolation prior to the series (In Prelude to the Stormlight Archive) therefore he maintained the Oathpact. They called him Stonesinew and his order that was associated with him was the Stonewards.
Tanat - 9 - is his spot in the Ars Arcanum and thats associated with Stone and the divine attributes of Dependability and Resolve. I would put it to you that in the books in his own twisted way with how he follows his orders that Szeth is incredibly dependable and stays resolved to his task. On page 430 (A matter of Perspective) the epigram from Words of Radiance, points out that those who followed Talenelat were also known to be very stubborn even in the face of proven error (like say a surge binder nascent-radiant in front of you?).
Because Talenal remained with the Oathpact part of the world was not given excessive investure because that part (Talenal's part) is still be invested in the Oathpact so the area still (in addition to the mountains) has 'normal' plants and animals.
Tanat is associated with Rocks and Stone and Stone Shamanism is said at the end of Words of Radiance by Nalan to specifically worship the Spren of stone - those who would be granting the Stonewards their abilities presumably or are thought to be mistakenly by the Shin.
The Honorblades were thus trusted to those who followed Tanat because he still followed the oathpact (alternatively they gathered them to protect them. Spren as we know considered the Honorblades dangerous as there was no 'check' on it) and their stubborn insistence on their being no voidbringers and no knights radiant would be about the fact that they 'won' thus anyone who claims otherwise would be doubting the spirits who told them that.
As for why they consider fighting to be a lesser art as those who once followed the Harold of War? I like it as a juxtaposition of what seems to have happened with the other Harolds. They have twisted their beliefs and were broken - the Shin have twisted the job of Tanat believing it was all over.
I'm a bit pressed for time else I'd pull some quotes but if you have any thoughts or questions, I'll do my best to respond!