Because spinning a coal plant up or down is slower, the batteries let you make the gradient more gentle.

Suppose Alice's gas plant can go from 100MW output to zero in ten minutes. So if Sam's solar farm goes from zero to 100MW over ten minutes in early morning Alice's plant can mirror, between them Alice and Sam produce 100MW of power smoothly. Obviously real life is more complicated but this gets the point across

Now, Charlie owns a coal power plant, it can ramp from 100MW to zero, but it takes about an hour. This doesn't directly mirror the solar farm, Charlie and Sam can't really just work together to smoothly produce the same power

Bill's 100MWh BESS fixes this. An hour before the sun comes up, Charlie's coal plant begins shutting down, Bill's BESS puts out 100MW while consuming the gradually reducing coal power output and some of the gradually increasing additional power from the solar farm as that increases.

This is true only in the sense that fossil fuels, as a percentage of total energy consumption, are stable or in slight decline. That is because total energy consumption is rapidly increasing, specifically as new forms of energy are added to the mix.

Fossil fuels in absolute terms are, however, increasing.

China is burning more coal today than ever before. Coal use is going up. Coal use as a percentage of total energy is going down.

China is burning more oil and natgas than ever before. It is increasing. But as a percentage of total energy, it is decreasing.

This also matches China's CO2 targets. China's plan is not to decrease CO2 emissions, those emissions will keep increasing. The targets are to decrease the emissions required to generate a dollar of GDP. By 2060, they want the ratio of emissions to GDP to be 60% lower than it was in 2020. If you look at historical trendlines, that is an easy target to meet, just with improvements in technology and energy efficiency, particularly when you are starting from such a high base. But people look at that goal and think that China wants to cut emissions by 60%. GDP in 2060 will be 10x what the GDP was in 2020, so a 60% cut in that ratio is still a 400% increase in total emissions. Everything is measured in terms of ratios, not absolute numbers. Ratios where the base is always rapidly growing.

This confuses a lot of people who see that the ratio of fossil fuel in the entire energy mix is decreasing, and think this means that actual fossil fuel use is decreasing. The charts, report, graphs that people cite, if you look carefully, are always ratios. Ratios with the denominator growing as the denominator is something tied to GDP, whether total output or total energy output.

It's more accurate to say that there are limits to how much additional fossil fuel capacity China can bring online, as these are mature industries, whereas nuclear, solar, etc, are young industries and there is a lot of capacity to bring online since you are starting from a low base. China's approach to energy is always "we want more of it, of any kind." The only reason that China's oil production is not increasing by 10% per year is because it's not possible for China to increase oil production that quickly. Same for coal. China is increasing all forms of energy as fast as possible.