Isn't the point of prison to separate criminals from the general population?
This; but with some very important caveats. First, the separation must be as humane as practical. Why? Well there's the obvious "don't be a sicko and mistreat anybody, even a prisoner because it's immoral" argument. There's also the "hey, they might be innocent" argument. Everybody, even the most hardened "tough on crime" person should understand this. Every single one of us is potentially a victim of mistaken imprisonment. What, not me? Yes. You. If you disagree, please refer back to the definition of "mistaken imprisonment". Thus, we all have a personal stake in safe, humane treatment of prisoners.
Second, we should carry out this humane separation only when necessary, and in a way that isn't too costly.
The policy as described in the article summary seems neither humane, nor necessary. In the long run it will be costly too. Extended solitary can lead to severe mental conditions. Severe mental conditions are costly to society, either because of new crimes when released, or the need to turn the prisoner over to an expensive psychiatric facility for the rest of their lives.
If they're concerned about people running gangs from prison, they could monitor their networks. Collect all the information and trade it with your friends... then collect their friends and put them in the same cell. Of course there'd be less of this if we weren't waging a war on drugs; but that's a separate rant.