because JUST using progmem at declaration of the array does not work .. more functionality needs to be implemented. That github snippet will never work without also implementing pgm_read_word() / pgm_read_byte() at the places the array is accessed.
Using PROGMEM is a two-step procedure. After getting the data into Flash memory, it requires special methods (functions), also defined in the pgmspace.h library, to read the data from program memory back into SRAM, so we can do something useful with it.