I stumbled upon an interesting Dīrgha Āgama passage which has a fire metaphor that specifically describes the death of an enlightened being. Referencing his parinibbāna, the Buddha says:
Buddhas, pratyeka buddhas, and [enlightened] disciples
All return to that same cessation.
They have no choice but be impermanent;
They’re like mountain forests on fire. (DA2, translation Patton)
This verse has no direct parallel in the Pali, but it shows how early Buddhist thought about fires. They believed them to be impermanent and to cease, also in context of parinibbāna. This understanding directly reflects MN28, mentioned in the essay, where wildfires are said to be impermanent.
MN146 likewise directly states that flames of oil lamps are impermanent, as a metaphor for the cessation of feelings. SN12.52 & 53 use the metaphor of the extinguished fire for the cessation of suffering. MN140 uses it for the cessations of feelings at parinibbāna, AN3.90 for the cessation of consciousness. In MN38 fires are said to burn in dependence on fuel. The Buddha then asks: “Do you see that if that fuel ceases, what has come to be is also bound to cease?” This is a simile for the cessation of consciousness.
The latter also illuminates Vacchagotta’s reply in MN72: “The fire burned in dependence on its fuel of grass and sticks. When that is used up, if it does not get any more fuel, being without fuel, it is reckoned as extinguished [i.e. as ceased].”
Not early, but for Buddhist doctrine still a better source than the Vedas, the Milindapañha says:
The king [Milinda] asked: “Venerable Nāgasena, is extinguishment cessation?”
“Yes, Your Majesty, extinguishment is cessation.” (Mil 3.4.8)
This is supported by the early suttas. For example, in AN10.60 the contemplation on nibbāna is called the perception of cessation.
Given the consistent use of the fire metaphor throughout the discourses, we can also conclude the Upasīva Sutta is about the “sage” ceasing and disappearing. (Aside from the clarity of the sutta itself, when translated properly.) Otherwise the Buddha wouldn’t have used this metaphor there, or he would very likely be misunderstood.