I was 100% just meming, but thank you for the serious response, you make a good point! 🙂

If you were already going to use the heat later in the day when fossil fuels are burning again, then whatever you can do to reduce that future consumption, through storing some thermal heat produced now, can still reduce that fossil fuel consumption overall. Water heaters, warming any living spaces that might need to be heated at night, etc.

It doesn't even have to be efficient when prices are literally negative. All it has to do is be somewhat effective at reducing later consumption.