See comments in Arduino Code:

// Teufel remote functions

// P0.0 Source switch
// P0.04 Power/Mute/Encoder C
// P0.01 Encoder A
// P0.03 Encoder B
// P0.09 LED2
// P0.29 LED1
// P0.13 RX - (unused)
// P0.14 TX - (unused)
// All inputs are pulled up, so active LOW

I just saw, the documentation of the pins is in the teufel.ino file.

Edit: Just finished to replicate the project. Works like a charm! I did'nt use the photoresistor to get the real current power state of the Teufel AVR, because the transmission of the commands is pretty reliable and there are separate commands for on and off. So now I finally again only have one (harmony) remote to control all of my devices.
BTW: Instead of an ESP32, I used an Arduino Nano with an Ethernet Shield and had to add the pairing function (>15s input-keypress) to make it work.
So last but not least, I want to thank you for your efforts! I also tried to analyse the protocol and therefore already had a 2G4M04S1D-Dev-Board laying around, but would never have had the idea to simply clone the binary from one NRF51822 to the other one. You saved me a ton of time! Thanks.

Hello, a very successful project that I would like to replicate. Could you document which pins of the NRF51822 you used? Thank you very much, Bulli223