That's a good point, I guess.
(however, the blog is on a page of a company, that seems to me to be selling network monitoring devices.)
Some performance management tools will use up some of the bandwidth of a network to be able to perform their duties. An out-of-band tap offers a way to get visibility into what's happening in your network so you can monitor performance without sapping bandwidth.
Because network tapping is fast becoming expensive, they provide the out-of-bands tapping as a service. An out of bands tap is like having a splitter on the wire that copies the signals that pass through the wire but does not interact with the data in any other way other than to copy it.
Attackers cannot see the out-of-band tap, but they can try to attack or detect its presence.
The infamous NSA tapping of AT&T's entire communication network in California was an out of bands tap. It copied all of the communication passing through the AT&T central comms node in California to a NSA controlled data site through an out-of-bands tap installed into AT&T's network in the form of a fiber-optics splitter.