"Powerwall" is just a big battery.

Yes... plus some electronics, like a rectifier, an inverter and control circuitry.

It might help compensate for the irregularities of solar and wind power.

Yes... that's partly what is was conceived to do. It also can compensate for the irregularities in demand.

But it only makes sense to use batteries for that after we have moved to much more renewables.

For storage of solar and wind power, this is a complex matter, and the short answer is "it depends". For demand management, it makes sense now.

It is much more efficient to store the power in the water magazines of existing hydro plants.

It is desirable to use hydro plants (where they exist) as swing producers. Pumped storage is not quite as energy-efficient as battery storage (roughly 75% for pumped vs 85% for Li batteries), though it can be cost-effective, in places where large, elevated reservoirs already exist.

But all this is besides the point, which is: There are proposals to "solve" global warming, which are implementable now, with today's technology, which furthermore have side effects which are on balance positive (like clean air in the cities).