If you have mountains, you have batteries. Pump water up the mountain when you have extra juice, pump it down when you need extra. I was also curious about a sort of spinning mass energy storage. Get a big cylinder of concrete or lead spinning with the extra juice, then generate the energy back.

You're thinking of flywheel storage. There are a few of these running. One pretty big one in Ontario for example.

Tangentially, some car manufacturers have done prototypes of this as well. Look up the Porsche GT3R Hybrid or the Volvo S60 prototype for an example... Unfortunately it appears that for one reason or another no one has decided to move forward with these into actual consumer cars.

There are a few reasons including weight, safety, cost, package size, etc. A flywheel that can hold appreciable energy requires high speed or high mass as well as a low drag / bearing friction environment. The passengers must be protected from catastrophic failure of the flywheel by some strong casing ( more weight ) and cant be 'turned off' in an emergency. In race cars where heavy braking and acceleration are the norm and cost is not a factor, light, strong, fast flywheels can do a good job provided they run on active bearings in a low pressure casing. In a heavy road car the additional weight required makes the engine less efficient in terms of mpg, and probably less safe during a crash ( or if similarly safe, much heavier due to casing requirements ). Combine this with the inability to externally charge the flywheel and there arent really any great reasons to use it over electric energy storage

There are very many people to whom it is not obvious that this only means electricity. Probably more than half the people that will see this headline, whether on HN or elsewhere. Think of all the environmental activists with no physics background who can't tell a Joule from a Watt or a Volt from an Amp. (There's nothing inherently wrong with not knowing but information should be presented so that it doesn't mislead, and people like that are probably part of the audience for the article.)

I'm sure there are a few, but you don't need a physics background, nor even to know the difference between Volts and Amps to realise that most cars on the road use petrol/gas. And that you or people you know likely pay a gas bill for your home heating.