The stats for all the way through kids are published, so you can look at the progress of those, if those show poor progress and poor results, then the "poor cohort" argument doesn't really stack up.
Obviously you need to look beyond the headline, but equally you rarely have sufficient information to look beyond the headline, but that is why so much of the job of the SLT is management of a school, which includes good communication, simply expressing faith is not great communication.
Personally I'd want to see a lot more evidence of failings than simply some poor SATs results, but I don't think it's unreasonable for people to expect more analysis from the head. The SATs information is public, and commenting it doesn't in any way breach confidentialities.
"We struggled this year to show good progress in maths, so we're looking carefully at the way we taught, and if there's any areas we could particularly improve having been given this heads up, I've asked "head of maths" to lead on this".
That is of course what the SLT is likely doing, but saying it provides so much better communication to parents who might be anxious - and disengaged parents, or moving children is not in the childrens or the schools interest unless there really is a reason.