They are buying electricity and storing it as heat and time-arbitrage it to when the heat is needed, they make no mention of the electric power source. In any case, during the depth of winter, when it's needed most, they're still burning carbon. Previous paragraph to your quote:
>During the coldest, most expensive stretch, the wood chip boiler became the primary unit, and the sand battery supplemented it.
Remarkably: heat is pointed to as "wasted energy" when doing EROEI analysis and discounted, this is done to strengthen the case for Solar vs Gas.
Finland's energy mix is ~6% solar [1]... maybe it's not a larger portion of the grid supply because Finns realize it doesn't work in the winter?
Finland only started building solar recently. Wind is still more cost-effective, if you only consider the cost of generation. But there is almost too much installed wind capacity. If you also consider the value of the generated energy, solar gets ahead, as it correlates less with existing generation.
In any case, Finland does not really use fossil fuels for electricity generation anymore. There is some cogeneration, where heat is the primary output, and reserve power plants that are only used in exceptional situations. Electricity is largely a solved problem, but it's proving harder to get rid of fossil fuels in heating and transportation.
going from burning fuel 12 months of the year to 3 is still a 75% cut in fuel costs and emissions