What about if this isn't the first invasion
perhaps there is an ancient civilisation of Abberations and the Chuul are coming back to reclaim it.
In fact with if the World was the original Chuul homeworld which in ancient times was invaded by advanced (Psionic) Humans. The original Chuul and Ultra-humans clashed and the ambient fall out casued some humans to degrade into goblinoids (who are swamp dwellers due to their Chuul heritage). Giants and Gnolls might also be races devolved from humans.
Anyway the older Abberation empire destroyed with the Abberations fleeing off into the Far realms. The Ultra-human civilisation spread but it to ewventually fell into ruin and was all but forgotten
Perhaps the 'Tibetan Culture' was lucky enough to stumble across Ultra-Human artifacts which allowed them to advance at a more rapid rate than the surrounding nations who are still neolithic
Perhaps the Chuul are in fact justified as they just want to come home

(and destroy any obstacle in an icky abberant manner)