The lower the voltage drop, the better. I guess, in their part of the world, germaniums are cheaper than schottkys
A regular silicon diode will not work. An appropriate Schottky silicon diode will do the job; see the Infineon app-note in the "PPS" of the website. Also, the point-contact (regardless of the semiconductor material) reduces the diode's junction capacitance, making it faster, i.e., more suited for RF applications.
It's a neat hack! Long live the Rigol DS1054Z!