Anybody who has been through education has observed that there are the people who "get it" and the people who struggle. And similarly, if you take an expert in some field and have them start explaining concepts to a random person that are at the very edge of the expert's knowledge, said person will be completely lost. This is a real thing we all can relate to.
The idea that "we all can do it" because "we're all equals" is doing a disservice to the kids who struggle. You better prepare them by putting them in courses that are matched to their level and challenge them appropriately.
You can't take someone who weighs 600 lbs, say "okay, go run a marathon" and expect success. Even if that person really wants to suceed. What you can do is train them in a way that is tailored to them, and eventually they may be able to run that marathon.
One idea I agree with is that we should evaluate kids more regularly to see how their abilities change over time. If they show improvement, their coursework should be adjusted accordingly. But all this "not sorting" kids stuff? That's just nonsense.