You may get a tty with no getty/shell running, you may get a getty login prompt (which was probably this case, as the author proceeds to login as root with an empty password), you may get a custom CLI that only allows you to set some device parameters but no arbitrary command execution.
You get a potentially bidirectional serial interface. Whether there is a shell running or any input accepted is another question. Very often they are output only, merely for debugging.
Comments
Not super familiar with hardware hacking. Does this essentially mean you plug in the write wires, run minicom, and get a terminal with a shell?
You may get a tty with no getty/shell running, you may get a getty login prompt (which was probably this case, as the author proceeds to login as root with an empty password), you may get a custom CLI that only allows you to set some device parameters but no arbitrary command execution.
You get a potentially bidirectional serial interface. Whether there is a shell running or any input accepted is another question. Very often they are output only, merely for debugging.