It seems complicated because there are multiple unions involved in the strike. If I've understood correctly, the big union (IF Metall) and Tesla have clashed over wage bargaining, but now a supporting strike involves also the employees working on the number plates.
This situation seems to involve the union/unions responsible for mail delivery, who refuse to deliver packages from Tesla, and by law all plates must be sent via mail.
So the suit relates to the state enforced monopoly on license delivery? That I suppose makes more sense, but I can also see the argument that the license is an official document and hence must be delivered by official mail.
Something vaguely similar is playing out in Texas. Paper plates are now illegal, and not without good reason (widely forged), but in the context of other laws passed at the same time it seems aimed at tesla. Tesla has no dealerships, it has been said dealerships don't work well with EVs because the lack of maintenance which sustains a dealership. In Texas you are required to have a dealership to sell cars. Briefly Tesla was going to transport the vehicles in TX to the new Mexico border and back before delivering them to buyers. Some accomodations were made to prevent that, but now the way to get plates in the absence of paper ones from the government is... Dealerships are allowed to keep plates on trade ins to put on new cars as temporary plates.