"VanWalleghem said there is room to push costs down as the company gains experience from these first few plants. The storage systems have a projected lifespan of about 50 years, which is an important data point when comparing it to battery systems, which have much shorter lives"

So both longevity and working towards reduced costs for future plants. I guess someone thinks the long-term costs will end up below Li-ion, and likely lower environmental impact, at least compared to the battery component.