Sweden had no lockdown and now they have herd immunity.
It is of little help to lock everyone down.
You model effectively says Sweden just lost 95% of its population.
exactly
The case of Sweden does suggest a lockdown wasn't needed, but they hardly have 'herd immunity'. Antibody testing in Sweden has found ~1 in 5 people in Stockholm have antibodies to Covid. There are several possibilities for this, including the possibility that the antibody testing is insensitive, but at this point we can't say they have herd immunity - or anything close.
Remember that coronaviruses cause ~15% of annual colds. We certainly do not have herd immunity to colds. These enveloped RNA viruses are very good at evading the immune system.