I wholeheartedly subscribe to this. I was never super smart, mildly above-average in both primary, secondary, high school and university too. You could say I always matched the challenge, but not much more.
All the time, there were people around me who were significantly smarter than me, grokking things easily, learning faster, remembering better. Ie my roommate during whole campus was a guy who passed whole high school without the need to study at home, he just remembered it all and did homework just before the classes. there were tons of guys like him. I had to study every day, not too much but consistently. He didn't perform much better than me re grades on university, but he didn't have to work as much.
But here lies the rub - I could easily surpass people like him, even better ones, by just putting in enough time and learn things. In many topics that didn't interest people like him, I was much better. Much smarter folks than me were fired from university since at some point stuff became too hard to just visit seminars and they completely lacked the habit to spent time to learn hard stuff, even by far the smartest guy in whole 120-member class had to put in significant time to learn it for exams to pass.
I kept working on myself, learned various languages (ie right now battling with french), re-discovered topics that forceful learning in high school made me resent (geography, history, biology, physics) and discovered stuff on my own by backpacking around the world (more history and how everything interacted, cultures, food, appreciation of people sometimes vilified back home like Muslims) and just generally stepping far away of my comfort zone for decade and a half, right after finishing university. Now when I look back (and/or meet some of them), I am much more whole and experienced person, when we meet and their kids come and show me some backward place on globe I tell them some stories I've experienced in that region and a bit about their culture and history (don't want to sound condescending, I love them, and my roommate remained my good friend and has very senior management position).
I completed backwardish unknown university and for last decade work among graduates form best European universities. There is very little difference amongst us.
Its truly what IIRC Chaplin said - success is 10% talent and 90% hard work.