That doesn't explain the difference in death rates. If Japan were turning away severe life-threatening cases (especially despite plenty of hospital capacity), you'd expect an increase in deaths and rioting in the streets – at the very least, _some_ indication that something was wrong.

And if they _weren't_ turning away life-threatening cases, you'd expect reports of hospitals stretched far beyond capacity.

There really aren't any other plausible explanations than "Japan actually has significantly fewer or less severe COVID-19 cases than expected". It would have to be an implausibly massive cover-up.