

The article says there’s a narrow corridor which is about 0.5-1 km wide. This means the Ukrainians are in operational encirclement.
Russians usually leave a small corridor open like this, because they’ve observed that Ukrainian high command dislikes retreating and insists on sending reinforcements and supplies, as they order units to hold to the last man. At the same time, they try to sneak out heavy equipment and elite units. Both resupplies and shuffling of troops/equipment along this narrow path are usually easy targets for UAVs, missiles and air strikes. That’s when they inflict the highest losses. This also softens the units caught in the pocket and makes capturing them easier and less dangerous.
Abattoir is just the french word for slaughterhouse, and it means the exact same thing. But point taken.