Hey, well that is actually NOT true!
Except of the federal state Bremen and Hamburg, foreigners can obtain a fishing rod licence without passing an exam. Again the regulations of the specific federal state differ in details. Some federal states require no fishing rod licence at all, whereas require a fishing rod licence for foreigners. Some federal states issue a German fishing rod licence without a proof of competence. However, many federal states require a proof of competence in the form of a membership of a foreign angling association or a foreign fishing rod licence. The validity of the issued fishing rod licence for foreigners depends on the regulations of the specific federal state and is between 28 days up to 1 year.
Find all information here:
http://fishinglicence.eu/germany
Hope this helps!
Cheers,
Seb