I think the explanation for this is that Seniors and those with immunocompromised systems will lock down themselves without being told. And, locking down young healthy people changes nothing.
The Johns Hopkins study on lockdowns has serious flaws. For example, it is a “meta analysis” or study of studies. The authors found more than a thousand studies about lockdowns and excluded all but 34, for various reasons. They ignore the fact that because lockdowns were a response to serious rapid Covid outbreaks, those areas with lockdown policies have more Covid, but that doesn’t mean the lockdowns didn’t prevent more serious or more rapid outbreaks. A study of medical practices by economists is no more valid than a study of the economy done by doctors.