Party-line / Full-duplex intercom - mixing audio signals
No, that's not what he does. He feeds each microphone through an amplifier then through an output impedance that feeds the line through R14 and C4 shown in thick red circle below: -



I've shown the wire that is the actual line used by the intercom in a thin red/brown circle.

Because of the output impedances of several units connected all at once there will be no clash of signals and, within reason, the line voltage will be an approximate sum of the contributions of all active intercoms.

AC coupling via C4 is needed for the op-amps but note that there is a call circuit that needs to put DC onto the line to "ring" the other intercoms.

Side-tone cancellation takes place around U2B and for a 1-to-1 call you won't get feedback in your ear of your own voice. With several of these units connecting at once you may get feedback from mic to ear and this could cause havoc to other users too. So be aware of this.

Balanced Line out with DC signalling: -