Nuclear is incredibly energy dense and reactors have a very long lifespan, so it makes sense that decommissioning it would be cheaper than solar panels. The 1.6 GW reactor in Finland has an operational lifespan of at least 60 years, whereas solar panels currently last 20-30 years. Given that they last half the time and that a 1.6 GW solar installation would be absolutely massive (something like 40 km²), it stands to reason that solar would create more CO2e/kWh to decommission.