It seems doubtful they’d win in cassation as the GDPR does literally say that personal data shall be accurate (article 5 1.d) and that personal data subjects have a right of rectification (article 14).
Their only possible argument would be that fixing their shit is not a “reasonable step”.
I’m not sure that even falls under the purview of the cour de cassation, but IANAL so I’m not quite clear on what a “matter of law” is exactly.
I’m sure it will be expensive.