They are the flesh, etc. The prophet is led to retort their derisive or defiant words. Not they, but the carcases of their victims, were as the "flesh" in the "caldron." For themselves, there was another fate in reserve.
Neither to be protected by the caldron nor to meet their doom in it, but to be brought out of it. Death, by famine, sword, or pestilence (Ezekiel 5:12), might be the doom of some, but for others, perhaps specially for those whom the prophet addresses, there would be captivity first, and death from the sword which they feared, afterwards.