The question here IMAO is if they escaped by their own means or received external help. Similar history happened in other zoos subject to animalist harassment

The question here IMAO is if they escaped by their own means or received external help. Similar history happened in other zoos subject to animalist harassment (with "free the chimps" campaigns for months if I remember correctly). Finally the chimps escaped somehow in 2015. One was shooted and the other died drowned not much later. Maybe somebody naively expected a different outcome or even still regret to participate in that campaigns. We'll never know. Chimps are known to be furry Houdinis in any case, but yes, they can be very dangerous.