You should follow the whole set of instructions as it creates the HEX file to upload. You can’t just pick and choose bits of a set of instructions.
Note the line “If the emonTX is connected to the RPi via the serial interface” (which perhaps could be clearer).
Also, you may need to edit the file /opt/openenergymonitor/avrdude-rpi/autoreset and change the pin number to 12.