M1 machines expose serial over Type C, so of course if you cross-connect two M1s they can talk to each other over serial. Conveniently, one of the pin pair options you can select is SBU1/2, which are cross-connected on standard cables, like a null modem serial cable, so you end up with TX connected to RX in both directions and it just works.
This is not a standard Type C feature, so unless another machine happens to also implement a 1.2V serial port compatible with Apple's version and pinout completely by chance, it won't work.
It only works on a specific port on Apple devices (the same one used for DFU, which has the fun debug features like this one), not on all the others.
