They are comparing it with building a new power plant costs for which are calculated similarly for 20-30 year life cycle. Same is for Solar and wind power plants. Many people assume that prices of grid scale solar and wind power are coming down because of technology improvements that is partly true but another reason for the price decreases is that the investors are finding out the calculated life of these is actually 50-100% more than their previous assumptions.
The other thing that is happening is that solar wind and battery are naturally working forward through cost-reduction manufacturing S-curve, while fossil fuel plant tech has already hit the mature-flat portion of that curve. Renewables are basically at the point where there are easy & dramatic improvements in cost reduction by "just" volume scale up and financing, while fossil fuel plants have long explored those options and have more or less hit costs that are much more difficult to improve.
