Bubbles, the report kind of implies that deferring does not help SB's progress, but it does not have any basis for making that claim.
It has no basis for making any claim as to if/how deferring would help (or not) SB children.
For now, the vast majority of SB children are still in their 'correct' cohort, starting reception in the term beginning after their 4th birthday.
Very few parents even consider deferring their child's start. Of those that do, many discard the idea right away due to costs (additional year of childcare). For those for whom these costs don't matter, they continue to consider if deferring is actually in their child's best interest, seeing as it is not the 'done thing', the child would be in a very small minority, things are unclear regarding progression into next school stages/entrance exams/ what about moving area/moving schools?
If they still consider deferring, it is usually because their child is by all accounts simply not ready for school.
So only a small proportion of SB parents will even apply for deferral, and this will not be a random selection of all SBs - it will be those SBs who are significantly behind even their fellow SBs (in development), and whose parents can afford childcare costs.
Then of these, only a % of applications will be approved. In many councils, only those applications will be approved where there is professional evidence supporting the parents' view that the child is significantly behind their SB peers (developmentally). At age 3, not many will have that professional evidence, even if they ARE significantly lagging in their development. Only the 'worst cases' e.g. global developmental delay are likely to be able to provide such evidence.
So in effect, of the SB children studied in this report, only those did defer who were VERY behind (developmentally; on average).
So the report is comparing 'very behind' SB children to 'average-advanced' SB children. And finding that with the help of deferring, the 'very behind' SB children can (on average) keep up with their one year younger but 'average-advanced' SB peers (but still be at the bottom of the class/much worse than the average autumn borns)
It tells us nothing about what it would have been like for average-advanced SBs if they had deferred. Would that have improved their results? If the cut-off where in April (so effectively ALL SB 'deferred') would there be a new oldest-youngest statistical difference, this time comparing March borns with April borns? Presumably so, but we don't know, the report doesn't address this question.
Also it tells us nothing about what would have happened if the 'very behind' SBs had NOT deferred. It tries to imply that they would have achieved the same as 'average-advanced' SBs but that is a misleading implication lacking any basis in evidence. They do not have the data to make this conclusion.