Earlier quoted context omitted.
For that matter, why the optocoupler at all? You only need it if the systems are at different electrical potentials, and even then they are galvanically isolated on the Pis by the magnetics in the Ethernet. But I guess it's not the same as being asked "where's the air gap" pointing at the optocoupler, and saying "there it is"
There are lots of ways to solve this problem. In the past I have worked in defence, for highly sensitive stuff they wouldn't even allow a common ground between two networks. That's why I chose an option iolsator, it ensures the two devices are electrically isolated. It's overkill for this application, but I wanted to set something up right, and if I ever have another project like this that needs to be more secure, it…
I actually agree very much with this. If you're looking for strong assurance that there is no possible back-channel, devices like optocouplers help significantly. It's not hard for me to think of a way to surreptitiously send data backwards through a common ground, or normal silicon diode, or a magnetic coupler like an Ethernet transformer, but optoisolators make it significantly more challenging.