Herd immunity requires the members of the herd to mix, not isolate.
The concept has been used by farmers for decades, and it seems to work, but not by isolating infected animals.
The science is based on a statistical assumption that the higher the number of individuals who recover from the infection by developing natural immunity, the higher the chance their antibodies will transfer to others.
But it also requires that those lucky individuals who recover from infection are able to mix with everyone else so that there is an opportunity for their antibodies to jump from person to person; basically by exactly the same methods that the virus itself transfers – ie: in the air, surface to skin, skin to skin, etc. etc.
Personally I still think isolation sounds safer than infecting the herd, but hey, I am not a farmer !!!