Earlier quoted context omitted.
The know that presymtomatic people are highly contagious.
WHO disagree with you, so citation needed.
It's why track and trace is so important and difficult. People are contagious after 3 days but first show symptoms clustered around 5 days. So we need to contact trace their last week or so. A lot of western governments have been unable to do this so they have decided to hope presymptomatic people are not contagious and just ask symptomatic people to self isolate.
Another factor is what is symptomatic. I had a weird tightness in my chest 2 days before my fever and a tickle in my throat 1 day before. Breathing issues started 4 days after fever. I had no clue the tightness was covid...