You misunderstand. For it to be useful this winter, additional solar would have needed to have been deployed at the beginning of summer. Your timeline is: Russia invades February 24. Your most effective solar window is April-October. The solar would have needed to be built in 1-2 months to offset summer gas use.
It’s more effective to bring solar online sooner because it’s providing power every month after being installed thus providing a longer window to stockpile natural gas. However, while output is lower in December it’s still useful for December, January, and February and then continues to provide power into the future.
The EU is very much still concerned about having enough natural gas for next winter.