It would be due to multiple variables. There needs to a certain amount of bacteria in the small intestine. Some of the variables would be, the amount of bacteria initially ingested, number of bacteria that make it past the stomach, immune system of the host, time for the bacteria in the intestine to reach a concentration to cause symptoms.
Probably a lack of good information - with only a hundred odd cases and not having had much time to get the facts, I'm betting that's a SWAG (scientific wild ass guess) and we'll have better info the more time has passed.
There are multiple different types of salmonella infections. They typical serotype in Africa is invasive, meaning it's in the blood and people can become septic. This one appears to be non-invasive, it is similar to food poisoning.
And the latter being the article being based on? Makes sense, thank you! You'd still expect most of the people die due the invasive strain, but its probably people with abnormal immune response such as elderly and immuno compromised people.