Falacia translates to "fallacy," meaning a mistaken belief, especially one based on unsound argument. It's a more sophisticated word than typical song vocabulary, making it stand out.
In the song, Nio Garcia uses it to express disillusionment, singing, "El 'felices para siempre' era falacia" (The 'happily ever after' was a fallacy). This reveals a deeper emotional complexity, as he realizes the romantic ideal he once believed in was false, adding a poignant layer to his heartbreak.
Nio Garcia's "Autor" is a really emotional track about a love lost, where the singer feels like he's been relegated to a supporting role in his own life story. He's heartbroken, saying, "La vida me quitado las ganas de querer vivirla / Ya que no es contigo" [Life has taken away my desire to live it / Since it's not with you]. He feels like the 'happily ever after' he imagined with his love was a lie, and now he's forced to watch her with someone else.
He describes himself as the "Autor de cuento de otro" [Author of someone else's story], condemned to write her history with another man. It's a powerful image of feeling like an "antagonista" [antagonist] in his own narrative, with the painful price of seeing her with someone new. He repeatedly pleads, "Ay mami, no me dejes solo" [Oh mommy, don't leave me alone], showing just how much he misses her and how broken he feels.