Today you get a 
Apparently about 2/3 of the population already had the resistant traits.
If they had had to wait for the resistant traits to "evolve", then mortality in the exposed population would have been nearer 3/3.
No 1/3 already had resistance, 2/3 died leaving the the 1/3 “strongest” in relation to this selection pressure to survive as the “fittest” . Evolution was a combination of the variation that led to the resistance coupled with the selection pressure of the disease acting on the entire population leaving only the resistant population to procreate and pass along their resistance to the next generation.
Apparently thinning the herd for hundreds, maybe thousands of years.
A number of things like that is likely going on with us presently.
Who would have guessed that cancer is contagious...wouldn't that be something, eh?
Today you get a
If they had had to wait for the resistant traits to "evolve", then mortality in the exposed population would have been nearer 3/3.
They had the particular mutations because evolution is going on constantly. In the absence of selective pressures, the mutations tend to remain at fairly constant levels throughout the population. (That is, if 5% have the mutation in an initial population, about 5% of each subsequent generation will also have the mutation.) When selective pressures are added, giving those with the mutation a survival advantage, the incidence of the mutation throughout the population increases.
In this case, there were already selective pressures on the population to spread this particular mutation (there were lots of infectious diseases in Europe), which is why 2/3 of the population already had it when the Black Plague hit.
In other words, what is described in the article is exactly what we expect to see happen in an evolutionary scenario.