They might be tricking the GBA into thinking it’s a GB cartridge (it has a built-in GBC “emulator”). I found different ways this could work online: perhaps by adding extra plastic to actuate a switch (that switches between 5 V (GBC) and 3.3 V (GBA) power to the cartridge); alternatively, the ROM chip is made to withstand both voltages and the system selection is done by a 0 in the top bit of the cartridge’s “B” register.