Having two distinct routers and physical devices just isolates each network that much more. A single device is still exactly that at some point, and in such cases there is always the possibility of an exploit that could compromise the device fundamentally.

I think the author is just advocating for a very locked down approach but I agree it is not feasible for most folks.

The author is proposing a solution that is a naive implementation of an existing technology: VLANs.

This is what you want to do:

* Isolate each group of devices with different VLANs

* Use firewall rules on your layer 3 device (router, firewall, frankenmodem, etc.) to prevent communication between the subnets tied to those VLANs.

This is an example of what this looks like at layer 3: https://documentation.meraki.com/MR/Firewall_and_Traffic_Sha...

because they dont understand how networking works.