I then went to a decent university (a French Grande Ecole) and for the first year, I was still doing okish but I went from being a student who always had good results to doing average. Then, the second year came, and just understanding the concepts without cultivating fluency by practicing wasn't enough anymore and I failed hard. I hadn't learned how to learn and while I understood the concepts and could easily follow when the teacher solved a problem together with us, I had difficulties doing it on my own. I worked doing exercises and was able to pass but time and time again in my professional life, I've had the same issue where I tend to rely too much on my intuitive understanding until it fails.
I'm not sure how to help kids not to go down this path. As a kid, I would avoid any rote memorization and would probably not do any non graded exercise since I knew that I understood (and also since as a geek wanting to fit in, I tried to be more accepted by my peers by purposefully not doing my homework). In the end, I think the key is play and giving maybe challenging exercises that forces the kid to use his newly found understanding would be a way to get them used to not just rely on their intuition understanding.