========They have also observed that genetic factors in the population are preventing specific components of the virus from destroying the host cells.

This is mistake in translation, mistake of understanding or both. Viruses what destroy cell doesn't exists at all. Cell is home and castle of virus, nobody destroys own home. About genetic factor is also speculation.

Real fact is: in Japanese subline of Delta stamm occurred mutation of chain of molecules what prevents virus's replication. So Japan got virus-impotent, that can't replicate itself.

Unfortunately this "miracle" will not help to Japan for long time: when population of mutated viruses will die, that place will be taken by next one mutation from another region, unless Japan will tightly close it borders as China did.