Kids have no perception of time at all! Heck even adults struggle to report the duration of a traumatic event as when something awful happens time seems to stand still and we lose all sense of time. Your son says it was 10 minutes so you automatically believe him instead of engaging common sense and thinking, hang on a minute, this doesn't sound right.
My point in both responses to you, and the OP, is that you can't always take a child's word for what happened as gospel. I'm not suggesting that anyone is lying, in either circumstances. It's just that the facts as told by a child, from their perspective, can be very different to the reality.
What you describe could have taken place in a minute before staff witnessed or were alerted and intervened. If a child was being 'strangled' for 10 minutes, they'd be dead! Common sense alone should be enough to tell you that's not quite right.
Yes I am a teacher and therefore I know that no incident would have persisted for 10 minutes, to that level of reported violence, without adult intervention in a school playground. Despite what Mumsnet would have you believe, playgrounds are supervised. If staff hadn't witnessed something like this themselves straight away, and had to be alerted, the likely explanation is that they were already dealing with something else such as first aid or another behaviour incident. It won't be because there was nobody out there supervising!
As I said to the OP, terms like 'strangled' can be misinterpreted by children. My point has always been, parents need to check the facts with school rather than simply taking their child's word for it on everything. By nature, a young child's account of an event can be unreliable. Again, not because they are lying. Their perception can often be different to the reality, that's all.
I'm just trying to add some common sense, among the hysteria.