This is an interesting question, and depends on how readily available battery materials are, and what environmental harm is done by mining them rapidly.
For example, it's possible that Toyota can use X amount of battery to increase the mileage on 10 hybrids from 30 mpg to 40 mpg, whereas a single EV would use the same amount of battery and increase the gas mileage from 30 mpg to infinity. Depending on how much that one EV is driven, versus the 10 hybrids, it could reduce emissions further to go the hybrid route.
An alternative hypothesis: Toyota loves just-in-time delivery in its manufacturing process. Maybe they're doing the same thing here, at a larger scale. Once there are plenty of charging stations and consumer demand, they'll build EVs (but no sooner!).