Its a partial description of your IT farm.
The article is pretty accurate but I don't agree that missing a device automatically invalidates your monitoring, most everything else is still valid but things that might touch or be touched by the rogue entity are potentially compromised. Always shades of grey.
What matters there is that when something is added to the farm you have a way of seeing it, whether it is IP discovery scans, DNS additions, new mac addresses appearing in DHCP or whatever so you can track it down and absorb it.
When adding a metric I've already gone past the is it needed phase and look at how it is to do its job, is it a binary check, simple warning/critical thresholds or is it something more contextual, all to try and avoid false positives.
Once in then like every other alert it gets tested every few months manually to make sure it still works but if an alert is working and generating alerts which are not getting resolved then the alert itself is challenged as not needed or incorrectly bounded. Where possible alerts should always be an exception.
I usually joke that if I'm doing my job well enough then I've just done myself out of a job because everything important is now covered and there is nothing really for me to add.