It doesn't mean 'normal' as in common, it means normal as in 'not abnormal'. I agree 13 is a young min age (it relates to 13 to 17 year olds) but ultimately, a 13 year old having sex does in itself indicate mh problems, abuse etc.
Its not great, I would still be concerned about that child as it is not safe behaviour. I do not believe 13 year olds are emotionally ready. But it has been taken out of context and used as a stick to beat 'sex education' with.
This tool is for helping professionals spot children who have abnormal sexual behaviours and highlight that, like it or not, children are not asexual. Some 13 year olds have been sexually mature for some time, given that puberty can start at 8.