They don't have many buses like that yet and buses are always near population centers where using food waste to make biogas is simply much better in almost every way.

As for storing excess energy sure if we're talking solar generation but they use a lot of hydrogen too for this project and in that case pumping water up to the dam is a much easier and probably more efficient than generating hydrogen and either using it to run an engine or store in fuel cells. Fuel cells aren't all that efficient. Overall a lot of money spent that will not at all pay for itself for that use case.

I really struggle with why they've gone so heavily into generating hydrogen when there is a big lack of viable use cases. Though they're far from alone in overestimating hydrogen, BMW and Toyota both invested heavily in fuel cell research (and BMW experimented with direct hydrogen use) and neither came out the other end a winner.