Originally, it was radio only. I think serial was added at some point, I know there were some complaints because the radio was left on as well as serial when only serial was wanted. I think the “either - or” happened when whitening was introduced.
What’s missing from the emonTx is sufficient I/O to be able to have a bank of DIP switches to be able to select all the options that it’s accrued over the years.
You will still use your ESP8266 once you have changed the output of the emonTx over. You will need to disconnect the ESP to do it, check that the serial output is present with the correct format, then reconnect the ESP, and you should see your output.
It might be possible to use another serial comms program to talk to the programmer, but the simplest is probably the Arduino IDE, which will be useful if you want to update or change something else. Instructions for setting that up are in the ‘Learn’ section. You won’t need any of the libraries until you come to change the sketch itself and recompile and reload it.