What is the sense of the word "kettle" here?

It's a strategic formation. It's normally kettling, and I suppose "kettle" is the noun form of it. It's described thus:

Kettling (also known as containment or corralling) is a police tactic for controlling large crowds during demonstrations or protests. It involves the formation of large cordons of police officers who then move to contain a crowd within a limited area.

The police formation looks like a "kettle".

The link I gave has several instances of kettling described for further detail.

Kettling (also known as containment or corralling) is a police tactic for controlling large crowds during demonstrations or protests. It involves the formation of large cordons of police officers who then move to contain a crowd within a limited area. Protesters are left only one choice of exit, determined by the police, or are completely prevented from leaving.

Wikipedia

According to the BBC the term is derived from the German military usage of the word Kessel (cauldron) meaning an encircled military force, for example in the Keil und Kessel tactic used extensively against the Soviet Army in WWII.

The word derives from the German word "kessel" - literally a cauldron, or kettle - to describe an encircled army about to be annihilated by a superior force. For soldiers within the kettle the situation would soon become unbearable hot.

Kettling is also what birds do when they wheel in a circle on a thermal column of rising air. It’s like a free elevator for them. From Wikipedia:

A kettle is a term that birdwatchers use to describe a group of birds wheeling and circling in the air. The kettle may be composed of several different species at the same time. Nature photographer M. Timothy O'Keefe theorizes that the word derives from the appearance of birds circling tightly in a thermal updraft "like something boiling in a cauldron." Osprey-watcher David Gessner, however, cites a Pennsylvania lowland called the Kettle ("der Kessel" in Pennsylvania Dutch), near Hawk Mountain, as the source of the term.

In some species—e.g. the terns of Nantucket—kettling behavior is evidently a way of "staging" a flock in readiness for migration. Pre-migrational turkey vultures kettle by the hundreds in the thermals that rise over Vancouver Island before they venture across the Strait of Juan de Fuca toward Washington State. At Hawk Mountain, broadwinged hawks form kettles in September before flying south. Kettling apparently serves as a form of avian communication—an announcement of imminent departure—as well as a way of gaining altitude and conserving strength. Because of this they kettle a lot.

You see turkey vultures kettling all the time, circling on updrafts. Terns and gulls do this a lot. If you’re lucky you’ll see hawks, kites, or even sandhill cranes doing it when they’re in or prepping for migration. It’s an awesome sight. Google for kettling hawks or kettling birds to see what I mean.

At some level, probably that of the underlying Kessel, this seems somewhat related to the policing tactic.