The legal matter is clear:
Amazon does not have to join a trade association (Unternehmerverband) and does not have to deal with a union. The state has clear non interference rules. And there is always the question wether verdi actually IS "Tariffähig" (empowered to run negotiations) when it comes to Amazon (and logistics in general) since they have a less than 10 percent membership rate with Amazon Logistics. And according to laws a union must be strong enough to fight through for its members (Mächtigkeitsprinzip).
That is one of the reasons why verdi does NOT try to negotiate a better logistics tarrif, the big companies like OTTO/Hermes might decide to ask the "Empowerment" question and kick out verdi completely. And then they might look at other contracts (OTTO/Hermes is partially logistics and partially retail tarif)
So there is no case for the court. The workers have a right to join verdi (for what it is worth) and go on strike and Amazon has the right to ignore them, hire a few thousand extra temps and/or redirect traffic to Poland.
The long term effects are not part of the law currently. That is due to the fact that union in germany until the 1990s had two forms
IG's with a 80+ percent membership and the power to fight through
They knew that too much/too long would cost jobs. AND they had to pay a lot of strikers (Union pays it's members "wages" (Streikgeld) when the employer sais "workers go home" / Lockout due to a strike). So strikes where short, last resort and to the point. And they always had a good idea "what can we get, where will the company stop"
The unions that became verdi that never had to strike because the state payed
That changed in the 1990s when the ÖTV (public service and transport) garbage men went on strike and the state said "go away". End result was a loss of support after three weeks of no garbage service, an acceptance of a lousy deal and the femal boss of ÖTV running away to Brussels.
Never the less the verdi guys never learned how to deal on equal level or even on a lower level with their employers and it still shows.