I've had a flick through the study and while it makes the case that kids for the next few years will have suffered learning loss, they absolutely do not make the case that this will translate to poorer GCSE grades.
Children who miss a lot of school perform worse at GCSE, yes. But if the whole cohort misses a lot of school, and performs worse at GCSE, the key question is 'will that mean that fewer of them pass?'.
And the answer to that is 'it depends on whether the government says they do or not'.
Grade boundaries are set by something called comparable outcomes - in order to avoid grade inflation it was decided that the proportion of kids getting a grade 4+ each year would stay roughly the same. Grade boundaries are set to achieve this.
There is a slight allowance for the ability of a cohort - if a cohort is brighter than previous years, then adjustments can be made so that more of them pass, or the other way around. Previously the ability of the cohort was decided by looking at KS2 SATs results. More recently this has been assessed by something called National Reference Tests which are sat each year by a random sample of Y11s. It's the same test each year so allows for cohort comparison.
Last year it was decided that the grade distribution would return to 2019 levels - so the same proportion of kids should e.g. get a grade 4+ in maths as did in 2019. The national reference tests showed that last year's Y11 were actually worse at maths than previous cohorts. Really there should have been an adjustment to account for this so that fewer of them passed, but it was decided that there wouldn't be.
So basically, 'worst GCSE results for decades' will be a government decision and I doubt they'll go for it.