Here's a pdf on the process: http://www.avfallsverige.se/fileadmin/uploads/forbranning_en...
No, this is not an example of a sunken costs fallacy. This is not "in for the penny, in for the pound" sort of logic.
Consider that the alternative is letting the organic waste rot, and in the process, release C02 and methane.
What they are doing here is removing CO2 temporarily from the atmosphere with solar power, then releasing the CO2 to produce heat which they then convert to electricity. It's solar power, they just use photosynthesis and the general population gets some use out of the middle step.
The reason this is not a good idea to do with CO2 that was taken out of the atmosphere millenia ago is that 1) we don't want that much CO2 back in the atmosphere, and 2) we don't want that much CO2 back in the atmosphere that fast. Doing it in the short term, unlike in the long term, is not creating "unplanned" conditions.