IMO school 'readiness' has little to nothing to do with academic abilities such as being able to count, 'read', and any such things that are often seen as 'being ahead'.
Much more with developmental milestones such as developing 'theory of mind' (usually from age 4.5 onwards), and the social maturity needed to navigate large peer-groups, often minimally or unsupervised (especially important in reception, where often one group of children is working with the teacher, second group is working with TA, and everyone else is engaged in free-flow indoors/outdoors play and as such pretty much unsupervised - in addition to minimally supervised breaktimes/lunch).
The maturity needed for the type of learning typical for Y1 onwards, IMO, is on average only achieved at age 7 or so, and therefore more playbased learning in Y1 would benefit all children, not just SBs.
So I can understand that people with 'advanced' seeming children consider deferring. Just because you can read a bit already does not mean you're ready for school.