Moonrawr’s background explains how a Zandalari troll exists in pre-BFA Warcraft. There are isolated precendents of other races traveling (adventurers, wanderers, outcasts, etc.) such as Chen Stormstout in WC3.
Now Warcraft timelines, let alone lore, is a confounding thing especially when trying to put it into a believable narrative. Sometimes, you can get away with “a wizard did it”. The expansions are listed in some as being a mere 12 months apart (WoW being Y25, BC 26, and WoTLK 27 respectively.
In the story, set at the end of the Burning Crusade expansion, she is already a mercenary for the Burning Blade and a young adult troll (approx 17-18 per RPG sources on the topic of Troll age which basically mirrors humans).
(Y22) Prophet Zul sees the mists of Pandaria opening and a vision of the Thunder King reborn
(Y23) Zul appoints Khar’zul to promote his vision to return to Pandaria and warns the King of impending disaster. When no disaster presents itself, Zul is rebuked by the King and declares that trolls will work together to rebuild and overcome in adversity as they always have. They will not abandon their home. Moonrawr’s family becomes part of the cult of Khar’zul.
(Y24) Having received word of cultists, Rastakhan outlaws the cult of Khar’zul declaring them Enkwayan or unbelievers. Khar’zul remains distanced from Zul except by name and retreats from the capital after an uprising. This is part of Zul’s plan to weaken the king over time. He sends disciples to investigate the power of blood magic while Khar’zul provides a convenient distraction.
(Y25) Moonrawr’s family decide to leave the cult, knowing they cannot return to Zandalar or face the wrath of the king or exile in the Volduni desert, they flee on a sturdy boat built by her father. They arrive weeks later on the shores of the Echo Islands.
(Y25) Tolerated with some prejudice, they live on the outskirts of the troll community. An encroaching centaur attack kills Moonrawr’s family and most of the village. She is approached by recruiters of the Burning Blade for a hot meal and a warm bed.
(Y25-26) Moonrawr acts as a scout and mercenary for the Burning Blade in the Barrens and parts of Durotar.