Quarantine during a completely non-contagious period is obviously not needed. The standard protocol is to quarantine everyone who has come in contact with a suspected case. Since this would be unnecessary if the person were guaranteed not to be contagious, it follows that the "authorities" have concluded there is at least some chance of contagion during this period. I also suspect the quantity of virus shed is so low that the odds of infection during this time are low.
I suspect the "authorities" believe the public is incapable of making these kinds of distinctions, so the party line is simply that people during these times are not contagious.
Interestingly, the United States does NOT seem to be following the "quarantine everybody" approach that the rest of the world is following. I trust our CDC and our medical system far more than the WHO. Are we making a mistake? There is LOTS of food for thought here.