Old old old wired intercoms (1 master, N slaves) used 1 speaker for speaker+microphone in all the slaves. That’s all they were, in fact, the box with speaker and PTT switch (optionally a call-button). Plenty of circuits online showing how to do that.
Older less-efficient LEDs make better photodiodes than modern more efficient LEDs. In the latter, the emitting volume is thinner and nearer the surface to let more light escape, but provides very little “collecting” volume for incoming light. Like trying to catch photons with a butterfly net, and lousy at it, but where it’s sheer enough to let photons slip through easily when emitting light. The former had lots of that, and were able to collect more incoming photons to act better as a photodiode, but emitted light had a harder time getting out of the chip.
CdS cells (variable resistance) would emit yellow-green light when hit with enough high voltage… and only if DARK, else its resistance would be low enough for the HV to fry it instantly.
So… nothing earthshattering. It works, to a varying degree.