Children who know the correct names for their body parts and who know what is or isn't appropriate in terms of how those body parts should be treated both by themselves and others actually tend to be at lower risk of abuse than those who don't. Knowledge brings confidence and confident children are better equipped to speak up.
Little Timmy knows that his body is his own and that no one except him is allowed to touch it unless they have specific reasons to do so (e.g., a doctor). He knows about PANTS and that private parts are private, he also knows the names for his private parts. He knows what qualities make a good friend and the properties of a positive relationship. He knows about tricky people and what to do if he encounters a tricky person (we don't teach stranger danger now). He knows that if anyone - child or adult - makes him feel uncomfortable for any reason then he should talk about it with a trusted adult such as his mum/dad or his teacher.
Little Johnny doesn't know any of that.
Which child do you think is more likely to be targeted by an abuser and which child do you think is better equipped to recognise that something isn't right?
That's how doctors, child psychologists and social workers/services know a child has been abused. So if those 2 countries you mentioned allow this to occur, then if a child is actually sexually abused there is no way to tell.
Knowing about their bodies and the names of body parts is not a safeguarding concern. Knowing how babies are made is not a safeguarding alert. Safeguarding concerns are all about context.
Professionals don't know a child has been abused based purely on them having knowledge about sex. Yes, one of the things were taught to be aware of is a knowledge of sex that is not age appropriate but there is a world of difference between "my dad had a sperm and my mum had an egg and they put them together to make a baby and now I have a new brother!" (a real comment a child has made to me) and a child making explicit comments about sexual practices - there is context. I have had a child draw a picture of his family and in the picture the stick-figure mum had a bare breast with a second stick-figure standing directly beside it with an open-O mouth, sounds worrying except mum had recently had a baby and was breastfeeding so when he drew his family he drew mum feeding the baby - again, it's context.
Precocious sexual knowledge is just one warning sign of CSA and we are taught to look for others alongside it such as a child being withdrawn, emotional, showing inappropriate behaviours, using inappropriate language, bruising, injuries, infections, etc. because - once more - it's all looked at within context.