They draw a false conclusion....The only thing they can correctly say is that they were infected with Plasmodium sp when they died. The fact that they found two or more sp points toward chronic &/or recurrent infections, very common in that region at that time. A fulminant course of malaria leading to rapid death within days of exposure is less common than a more indolent course resulting in anemia and eventual organ failure. (Cf- Quartan Fever)
https://www.cdc.gov/malaria/hcp/clinical-features/index.html