Yes - I think the key thing is that "a necromancer" is something created by John for his resurrected world. John himself uses what might be called necromancy, but that's only an element of a broader set of powers.
What John is, pre-res, is actually pretty horrifying.
He's a regular human being, and something about him catches the attention of a vast and ancient entity entirely beyond human comprehension and communication. It picks up in some way that he is trying to help it, even though he does not understand that the thing he is trying to help isn't just a biosphere but a conscious, ensouled entity, albeit on a scale so vastly more complex than a human that they are scarcely comparable. It wants to help him to help it. They will both live to regret it.
This entity modifies him on a genetic level without his knowledge or consent - and we still don't know the extent of that. What can Gideon's abilities tell us about John pre-res, and how might they be relevant in ATN?
It's hard to say at what stage John not sleeping or eating went from a symptom of his mania to something preternatural. Maybe it was always both. He's certainly latterly manually tweaking biochemical processes to skip sleep and meals. What's not entirely clear is whether he's describing a situation where there's something else sustaining him, or if it's more akin to Harrow's experience on the Mithraeum and he is running on reserves he can't spare. Or whether the framework laid onto him by an entity so different to him has complicated these things for him - after all, even experiencing human bodily needs doesn't make Nona like eating.
Pre-res, he's operating at a level beyond the necromancers he created, but within a recognisable framework of energy exchange. : he kills those 100 people using the thanergy from five accidental deaths, and in GTN Harrow says "Give me a single death and I can go for ten minutes". But we don't know much of the broader mechanics of his abilities - just the aspects that he comes to identify as 'necromancy'.
John has been Changed and has no frame of reference for how or why. But he is working closely with cadavers, and much of his work will have involved the liminality of life and dormancy and death. So that's where his terrifying new senses take him. To the cadavers, to the workings of bodies, to what makes someone alive or not. But we know he had other abilities, and that he was aware of them: C- talks about the idea of him stabilising a glacier or manipulating the atmosphere. Later, we see him commanding water and earth.
He was given powers that could have reversed global warming. But he did not understand what was happening to him, rapidly deteriorated physically and mentally under the strain of how he was changed, and the entity that we meet as the King Undying does not possess a full human soul - his body contains some of his original soul and some of the earth's, and we know that the interactions between bodies and souls are complicated. Actually containing some of the entity that originally modified his body may account for the emperor seeming somewhat more stable (metaphysically, at least...) than the man he was latterly (and also, as you point out, makes the stoma recognise him as a Resurrection Beast).
