Yes, people occasionally die of it, but for this or any jihadist to think Y. pestis is an effective way to wipe unbelievers from the earth is naive. The range of antibiotics that take care of it is pretty readily available, and the "plague" bacteria are not currently "medically resistant" to them.
"When the microbe is injected in small mice, the symptoms of the disease should start to appear within 24 hours," is hardly rigorous clinical testing. You'd learn more about how to properly culture bacteria from any academic microbiology lab manual.
"Use small grenades with the virus, and throw them in closed areas like metros, soccer stadiums, or entertainment centers ... Best to do it next to the air-conditioning. It also can be used during suicide operations." Means they're spitballing this stuff. This is no more instructive than watching a half season of "24".