Usually it means "nameplate capacity", the maximum the unit is rated for, not what it actually produces during a year on average.

Edit to add: if you follow the link under the figure and see the "Preliminary Monthly Electric Generator Inventory", the Excel file that opens up has nameplate capacity as one of the key columns and I wouldn't be surprised if that were the source.

Also note, when calculating the economics (cost/benefit) of power plants, they do use LCOE (leveled cost of energy) which takes into account the cost of the plant over the energy it actually will produce, which would take into account those factors. But probably this specific metric in this article doesn't.