"hellfire something that ignores durability by nature"
No, it's just hellish fire, per definition. You can assume that it's more of a big deal than normal fire in verse, but without feats you can't assume anything else.
"I was under the impression that the "higher realms of incendiary" was meant to be interpreted in a way that it was out of this world e.g spiritual realms, not that anything 1 celsius above volcanic fire would burn them. "
First: Nobody is sayin that celsius above volcanic fire would do the trick, but per what it said anything significantly higher would.
Secondly: The description doesn't imply anything about that only supernatural fire would work. If it did, it would say that "While impervious to any source of heat from the human world, it can be harmed by the hellfire of Ifrit" or something along those lines. "Higher realms of incendiary" is just a fancy way of saying "Hotter fire".
"Also, be aware we're dealing with fiction here. Where we suspend our disbelief, such as being able to accept characters flying at faster than light speeds; with that said, I don't see the problem in believing a character may have heat attacks hot enough to negate absolute-zero."
Yes, but we can't fully suspend our disbelief. We have to analyze feats and be critical of things that can be common in fiction. Yes, we can accept FTL speeds and Absolute Zero (or even beyond it) in fiction, but we need quantifiable proof.
A simple "Lightspeed" statement without feats to back it up is not proof of anything, much like a random villain stating to be capable of "Destroying the earth / world / universe" without supporting feats is also not acceptable (This is surprisingly common in fiction).
For Frost's ice, we have to consider the information given: Its data says that it is "Colder than Absolute Zero", which in itself is dubious and regrettable. We need sufficient proof that something is Absolute Zero alone, and even more so to assume that something is even colder than it.
https://vsbattles.fandom.com/wiki/Absolute_Zero_Feats_in_Fictio
I recommend you read this page and analyze how Absolute Zero works in comparison to Frost's own ice attacks.
With actual Absolute Zero, the vibration motions of particles become either minimal or halt completely, and in the later case the atoms would lose all binding energy and fall apart under their own weight.
Not only is that not the case with the Frost, as they are simply ice demons who attack with blades, but all that their profile says is that "The air around their blades is beyond absolute zero". That's it, no more elaboration nor explanation.
Add that in conjunction that the narrative considers the Frosts enduring volcanic fire as impressive, but can be negated by unquantifiably "higher forms of incendiary", and I'm sorry, the Frosts are not beyond Absolute Zero nor does Dante have resistance to it.
