That article isn't about prisons, but city jails. Typically, the city jail is just the drunk tank & minor drug possession, right? This is for short term stuff; I don't think they keep anyone for more than a few days. You might drop some domestic violence or petty theft types there, but there isn't an established gang population.
Once you move up to county jail you'll get some people who are there longer term then state jail or prison after that. You'll have gangs in county jails & higher, but city jails are generally too small for that. They only hold a dozen people.