The Redditor is talking about a specific trap that some people who've been told from a young age that they're smart fall in to, at different ages. The trap involves - mostly - having not been challenged previously, and not knowing how to work through things that don't come easily.
When I was a kid, I spent 6 years at a school in Barbados, and some time at a school in Thailand, where I was unambiguously the smartest kid in the class, with the top grades, always. Then at 10 I went to the UK's largest prep school. I was behind in Latin, French, and a bunch of other subjects - especially those which required any rote learning (which I hadn't come across up to that point).
The transition to a school with /much/ better teaching saw me become a distinctly mediocre student almost straight away - faced with challenges, and faced with not being able to simply smart my way to the top of the class, I started rejecting work wholesale - something that stayed with me throughout the rest of my formal education, and led to me getting some truly shitty A-Level grades, to the disgust of teachers who saw much greater potential there.
Hard work and talent are both important, and so is the ability to encounter and handle challenge. If you grew up in an environment where you were coasting at the top, the first time you encounter serious challenge you might well fall apart.
I've recently started an MSc at what Americans would call an Ivy-League university (having talked my way without an undegrad degree), and the first module I'm working with is "Software Engineering Mathematics". Working through the chapters of the book I have consistently had real trouble getting to grips with the material. I want it to just fit in straight away - and I just want to understand it - and it doesn't, and that's a constant challenge.
Being a little older, and a little more mature, I'm throwing everything I have against it now, and seeing real dividends for perseverance. But every day is a struggle of fighting against the part of me that doesn't know how to deal with challenge, and I put that down to the sudden transition from coasting to the top to sudden massive challenge, and having not dealt well with it at the time - which it sounds like the very very original poster was also struggling with.