The article explains this. Think of it as an MLM scam or compound interest. The lethality rate just gives you a scalar increase on all the events that would have occurred. Lethality can go to infinity for all we care if transmission goes to zero. But even at low fatality rates, if everyone on earth gets it, that is a huge number of dead.

Think of two kinds of fires burning at different levels of a forest. The lethality could even go down, transmission rates could go up and still kill more people.