Six short lines, and almost everything in them is heat. The speaker says that arson is "as good as summer," which is a strange, unsettling comparison. Fire set on purpose and the warmest season are treated as equals, as if both simply burn and consume. The title, "Hyperthermia," means dangerous overheating of the body, so the poem feels feverish from the start.
Then the scene shrinks to a closet at noon, the hottest hour, where the speaker becomes "an urn." That image suggests ashes, a container for what remains, and it hints at a person who feels hollowed out or already half-dead. The last lines turn to the dead, who are "secure in their long-gone bed." There is a faint envy in that, I think: the dead are safe from the heat, while the speaker is still stuck in it.
The mood is airless, claustrophobic and eerie, with a strange calm sitting on top of the discomfort.