This makes sense when I relate it to my personal experience in high school and university mathematics. I took the advanced stream in high school, where we were introduced to calculus in Year 12, if I remember correctly. It was, as this paper describes, "significantly procedural": we learned how to do differentiation and integration, but we weren't taught what it meant. When I took mathematics (and programming, and analog electronics) at university, we had to apply calculus to real world engineering problems. The only useful things that I retained from high school was the notation and the handful of rules I'd learned by rote. (Power rule, chain rule, open vs. closed limits, etc.) I had to develop my intuitive understanding of calculus at university.
It doesn't make sense in my experience. I AP-tested out of calc 1 at college, for calc 2 I don't feel it was any different than what I had in high school, just going into more complex stuff. Still rote memorization, but I had no problem with applying it.
Without an intuitive understanding of mathematics, it's useless, because you won't recognise calculus solutions to problems out in the real world. Teaching students the procedure of solving calculus problems makes them very good at solving neat, pre-made calculus problems you put in front of them. but it doesn't teach them how to construct the calculus problem in the first place.
A failing I have seen throughout most education, especially in STEM areas.
Personally, I took calculus in Year 12 despite the fact that I didn't do any accelerated maths in middle school. The education system here just didn't offer it. I was taught maths in a heterogeneous class up until senior school, at which point students chose one of three different streams: easy maths, hard maths or a double dose of hard maths. So I can say from experience that there is nothing wrong with making middle school maths heterogeneous, since I've lived it and it seemed to work fine. It didn't stop anyone from becoming scientists or engineers.
(Different country, obviously, but Year 12 is still Year 12, and middle school is still middle school.)
When you make class too easy you hurt the good students by making them bored and not interested in it anymore.
