The brakes are rarely used, so they begin to rust. Having fully functional brakes are mandatory even if you rarely use them.

I suppose it depends on what parts are rusty and how rusty. Surface rust on rotors is not really problematic. I'm wondering if it's a subjective visual inspection vs a test of brake function.

Not just surface rust. Keep in mind that Germany salts the roads in winter, and water + salt causes corrosion in brake pads.

This. Surface rust on the rotors is a reason to fail the TÜV. Seldom used cars have the same issue.