How one persons immune defense reacts to the COVID-19 virus is very complicated and yet not fully understood.That is certainly a factor but as long as the DR has old people and lots of covid, it shouldn't make nearly that much of a difference.
Clearly something beyond us «experts» in this forum can explain.
Lots of factors, genetics seems to be one. Numerous others are on the radar. (previous dengue infections, sun/D-vitamin, etc. etc.). Will take a lot of future research to be conclusive.
For me, the important numbers I look for are number of deaths (above a «normal» year) and the capacity of hospitals for the really sick ones, beds occupied. Not remotely possible to make sense of other numbers when the amount of testing is at the low level as DR does.
DR is doing OK at the moment regarding the virus as far as I can see.