Agree, I'm missing some context to see why your parent post thought this was good enough. I suspect that "Lowest Demand" is being thought of in two different ways.

even at the lowest demand, a large, grid-connected battery can only provide minutes of back up power, and that too for only a fraction of already low demand.

A storage battery is not a viable means of balancing demand and supply over a time period longer than minutes.

What's needed is not more batteries but better transmission, more hydro where possible and where hydro is not possible, more nuclear.

When you're talking about lowest demand are you talking about for the whole grid? A city block? A building? A condo?

And I agree. I worked in solar and storage for a while. It's not a replacement, it's just an extension.

It works out ok for a battery system for a building but anything larger they just don't last long enough

Only the south australian grid, which is not a relatively large grid.

South Australia's population is small, under two million and it's demand fluctuates between 400 - 3000 MW. For comparison, the Canadian province of New Brunswick has a comparable peak demand but fewer than a million people.