A mechanic is certified to standards set by the regulating authority.
It is not for the mechanic to make judgments relative to airworthiness, only to comply with "the book".
Thank you ! Quod erat Demonstrandum.
(i.e. the mechanic didn't comply with the books and then made an unauthorized repair with an unauthorized material... whether or not the consequences were foreseeable is just a matter of semantics on your side of the Atlantic ocean and which in this matter we don't give a hoot to ).
Of course, the responsibility of the supervisor who presumably signed the chit off and the airline procedures are heavily engaged, and that's what tyhe verdict reflects, too.
Tha's how the Freench law saw it and that's how the verdict stays until the decision of the appellate court.
And it's tough that you don't like it. You've got to live with the fact that your law system doesn't get an application everywhere.
Tough indeed.