From the Wowpedia page it states that Talanji fully got on board with the truce thanks to a prisoners exchange that got Thrall an audience with Tyrande. He delivered Sira Moonwarden to the High Priestess thus getting the Kaldorei adhering to the Armistice.
I invite the actual book readers to confirm or correct me since yeah, Wowpedia page infos.
Together with an entire book’s worth of lore where Talanji and her followers are dealing with a surprisingly massive amount of dissidents that are increasingly openly opposing her rule and try to assassinate her, allying with Sylvanas loyalists in the process.
It is the Horde’s help in putting down those dissidents, the Widow’s Bite, that sees the Zandalari fully join the Horde instead of merely being allies of the Horde, and has her agree to the truce that includes exchanging Shaw for Sira Moonwarden.
Delaryn seems to have been more of a depressed hopeless thing when fighting. Doing it because they were bitter and twisted and just had no clue what else to do.
While Sira was a ragingly psychotic utterly deranged killing machine fuelled only by the need for violence.
I think Sira would need some really damn good therapy.