> I am no lawyer and I do not understand what is proprietary to Apple here, it just does not sound that big a deal to me.

The special technique is the IP. In hardware manufacturing, the process is the IP and can be considered a trade secret, so even if that supplier does metal finishing for a ton of different clients, Apple's specific "recipe" or whatever is a legally protected trade secret, they would not be allowed to do it for anyone else.

Apple is alleging a little more than going and asking the supplier to do it though, in the lawsuit they are alleging the former employee leveraged their own relationship with the supplier to present the request as if Apple had approved it (to use their wording "Misleading the partner to believe they had Apple's permission to do so").