The argument there is that the June cohort would have been stronger because it only contained Y11 kids and not also weak Y10 kids. However, that is not taking into account that the June cohort would have instead contained weak Y11 kids who would have normally sat early entry in November, who wouldn't normally have been part of the June cohort. I suspect that there are a lot more kids entered in November than the previous June who were affected by the pulling of early entry. This muddies the waters.
There's also the point that grade boundaries are set with an eye on KS2 results, so they would have taken the profile of the cohort into account.
Finally, the GCSE pass rate hasn't remained broadly the same, it increased by nearly 5%, which is quite significant. It has been around 58% for the last 4 years, but this year was 62.4%.
I am intrigued to know how the levels of progress at level 4 could decrease while the C+ level increased. There does appear to be something weird in the data, but I don't think this blog has gone into nearly enough detail in its analysis. Did the percentage of students on a level 4 remain consistent over the years?