It is shocking, but if you have a culture where staff believe these things don't happen and children are dismissed, most of the incidents won't make it to the reporting software. If the head believes it's not happening, he won't be asking about it.
As you said parental surveys are part of the OFSTED so that's when it comes to light. At my (not outstanding) school we are constantly preparing for OFSTED and regularly survey staff, parents and students, but outstanding schools often haven't felt the need because they weren't expecting to be inspected.
And there is still that attitude among some more old fashioned (experienced?) staff that it's just boys being boys/ all a bit of banter.
Long serving heads at schools that have "always" been outstanding and sought after are a unique breed. They have an untouchable smugness, of course their perfect school doesn't have these issues. Maybe it's not all, but it definitely applies to most I've met.
Younger/newer Heads are much more focused on safeguarding and very aware of the personal fall out for them if it goes wrong, but the long established ones at outstanding schools got used to the idea that they'd never be inspected again as long as their results remained good.
What's happened here, at every single school that went from Outstanding to RI is that the long standing head went on long term (paid) sick leave and then took early retirement. When they know the worst that can happen is they leave on a nice package..
Far from wanting to minimise it, as I've been accused here, I'm delighted that this is all coming out at schools like the one in OP.