This is likely intended to be use with batteries that can recharge from very low-energy sources, e.g. energy captured from the body. It has very small energy consumption in sleep mode, so the battery can charge between wake cycles. 200 nA is, indeed, very low.
Lithium thionyl chloride batteries can last 40 years, based on a self-discharge rate of 1% per year. They're used for things like sensors buried in concrete where it's a hassle to change the batteries.