4 ideas
As a child, the narrator Avice Benner Cho was made to perform an act in an abandoned restaurant: she was hurt in a small way in the dark, then ate what was given to her. This let the Ariekei speak of 'a girl who was hurt in darkness and ate what was given to her.' Because Language can only refer to things that literally exist or happened, the Hosts had to stage real events and use real people as figures of speech before they could say anything new.
The Ariekei speak with two mouths at once, and their words carry meaning only when a single mind with real intention stands behind them. A statement can therefore only express a thought the speaker actually has about a real referent. Signifier and signified are fused, so they cannot say what is false, and they cannot even use a word to stand in for something that has not happened.
The Ambassador EzRa is a mismatched pair whose two halves do not share one mind, so each utterance comes out slightly split against itself. Hearing it, the Ariekei get a jolt of impossible cognitive dissonance, which works on them like a narcotic, and they come to crave it until their society starts to break down. The point is that a mind built on perfect correspondence has no defenses against a signal that is internally split.
The Ariekei faction that trains itself to say false things, first in the Festival of Lies and later by deliberately using similes and metaphors, gains abstract thought, freedom from EzRa's addictive voice, and the ability to talk to humans without Ambassadors. On this view, the capacity for falsehood and metaphor is what makes concepts separable from things, and so it is the precondition of abstraction. Lying is treated as a cognitive achievement, not a moral failing, and it costs the speakers their old innocent relationship to the world.