That passage is most applicable to large networks with multiple subnets. With many devices to serve, not only would they benefit most from Content Caching, unless that Mac had a wired Ethernet connection that burden would need to be shared by its entire wireless network.
So if you have many devices (perhaps a dozen or more) you really ought to designate one Mac with a wired Ethernet connection directly to a router.
My iMac has an ethernet connection, but is also on my home wifi network. If I remove it from that network, I can't then connect my devices to the iMac.
That's ok. I think you mean if you were to remove it from the network (or shut it down) it could not serve as a content caching parent, which is correct. For small networks that spend most of their time idle it won't make a difference, with one big caveat: it assumes your wireless environment isn't particularly overloaded. If yours is such that reliable wireless connectivity is a challenge, CC would exacerbate it.
Oh and one more thing: on that Mac, set Ethernet over Wi-Fi in its Network Service Order. That way, if a viable Ethernet connection exists the Mac will choose it. Leave Wi-Fi enabled for all the many additional services that require it.
