Earlier quoted context omitted.
I don‘t see the causal relationship between the 100%ers getting 80% and the 0%ers suddenly getting 20%
If you slow down the pace to help the "zero percenters" and only cover 80% of the material in the allotted time, the students who could have handled 100% of the material will be limited to 80%. And that slowdown still won't be enough to help the slowest learners much, so they'll still only learn, say, 20% of the material.
In schools with blended ability levels, the worst students do better (pulled up by the higher students), but the best studends do worse.
Good if you are in the bottom, bad ify ou are in the top.