Modern culture discourages trusting first instincts about other people, treating snap judgments as rude or invalid — "it's not something that we value" — so people talk themselves out of an accurate initial read. Robert Greene locates the problem in social conditioning specifically, not in the instinct's unreliability: the suppression is cultural, which implies the instinct itself still works.
As evidence, Greene cites reader correspondence after The Art of Seduction: women wrote to him describing accurate bad gut feelings about men that they didn't trust at the time, entered the relationship anyway, and were later proven right the hard way. He ties this to a broader claim about human self-perception: "We are animals. We think we're all so intellectual" — people believe they're governed by the frontal cortex/intellect but actually operate on emotions, instinct, feeling, and sensation, which is exactly the channel a gut read comes through.
This is the trust-the-output side of the same skill whose input side is Body Language Reveals What Words Conceal and Every Action Is a Sign — instinct here is likely a fast, subconscious aggregation of those same signals rather than something mystical.