The point is they don’t have to proof if a piece of random data is indeed an encrypted blob.
But they do need to suspect it.
If they find an encrypted blob, ask for the decryption key, they decrypt the data and analyse the decrypted data, then they may not suspect that a different decryption key will reveal a different set of data.
I don’t know about Germany, but in Denmark (their neighbor to the north), you ask the employee for a statement from the doctor. The employer pays the doctors fee, and there are strict rules about when they can even ask for it.