What would be the difference between having a data diode between your control and monitoring network and external monitoring systems, versus just splitting the monitoring part off into a completely separate network with ordinary two-way traffic?
Monitoring systems are usually separate and often have their dedicated network too, but they still need some sort of network connection to your critical infrastructure to do their job (monitoring).
I was trying to explain that having a separate monitoring infra and network group wouldn't work as a replacement for unidirectional network setup, because you sill need to open network access between critical infra and the monitoring system in your design, which will expose it to the internet.
So like you said, you still need to have an unidirectional network in place.