It doesn't take into account the high hospitalisation rate and the fact that mortality is a function of hospital stress.
According to the PM, the chosen strategy should prevent both a super high hospitalization rate and a lengthy lock-down of the entire country.
Hospitalisation rate (and in particular admission to ICU) + mortality rate are strongly dependendent on age and health - no children have yet died of Coronavirus. The Italians have one of the most elderly populations in the world; in the UK all of the deaths have involved people who are elderly and / or infirm.
You seem to think the simple answer is blanket lockdown + quarantine. This can only delay (not stop) infection. And how long do you think your lockdown is sustainable when millions of people are going to be thrown out of work / unable to pay their bills and entire economies go into meltdown? Two months? Three at the most? What happens after that?