Herd Immunity in Japan, with few deaths?

This study suggests COVID might have spread very quickly in Tokyo, and that they may have reached herd immunity, with very few deaths.  

...and with mass mask-wearing, perhaps suggesting that they're not particularly effective?

What might've caused the virus to have such a minor impact in Japan, compared to some other countries? 

They eat a lot of fermented food; could be a factor at least! Others have suggested this. The Japanese diet in general is known to be very healthy - they have one of the lowest obesity rates in the world.