That doesn’t mean it’s unenforceable. You don’t need a permit to leave Germany to any country as long as your planned stay is shorter than 3 month. The only way this could be enforced is by checking if people are in country, that is in case of drafting them. The paragraph essentially ensures that any person that gets drafted needs to present themselves in person within 3 month of the draft notice.

EU law overrides and breaks German law. Germany is d'accord with that.

So if german consitution sayed, starting in cold war era, what this law states, then the newer joining into EU made a new law, bringing freizügigkeit ("feedom of movement") to superseed even our Grundgesetz.