That's a mystery

Shogo Aoki, a Japanese researcher, said that by threatening North Korea recently, Trump was putting America’s interests over those of North Korea’s more vulnerable neighbors.

“I think (Trump) is thinking that this won’t result in any deaths back home, and if a war happens it will be far from home,” Aoki said. “Whether or not he's thinking about other countries -- Japan or Korea -- well, that's a mystery.

“In the worst-case scenario, a missile could drop on Japan, and I am very worried about that,” he said.

Wha does "that's a mystery" mean?