How do metal pipes amplify radio time signal?
It sounds to me like the addition of a ferromagnetic material will actually “focus” the magnetic part of the EM signal produced by the German radio transmitter. According to wiki it transmits around 80 kHz and, at that frequency, most random pieces of iron or steel will concentrate the magnetic part of the transmission just like a regular ferrite rod antenna: -



In concentrating that field, the internal coil used to pick up this transmission will be given more magnetic flux and, more flux means more induced “antenna” voltage and a stronger received signal.