While the energy source of wind and solar are free, building and maintaining the plants is not free.
If all they were building were a single 100MW data center, sure: renewables would be cheaper to build. But they evidently are planning a major campus that will consume most of the plant's 615 MW output over the next 25 years.
A solar installation, in a favorable location, capable of supplying 600MW around the clock (with battery backup) would have to be roughly 1800MW in capacity. It would be among the largest inthe world, on the order of ten thousand acres in size at a build cost of maybe 2 billion. The battery backup system to ensure 99.9% uptime would be 9x the size of the largest li-ion grid storage system ever built, and set you back on the order of 4-5 billion dollars.
While it's probably cheaper to go renewable than build a *new* nuclear plant, if you can reactivate and one for just two billion that looks like a bargain, if you have a use for all that power. Inability to save money by load following is the financial Achilles' heel of this generation of reactors, but if you have a guaranteed customer for most of your output, years in advance, that's as close to an ideal economic case for them as anything could be.