They aren't being pedantic, they are just flat wrong. To give a little more context to this, the wording on Staff of Focus says "In addition, if the bearer casts a spell that inflicts any mortal wounds during that phase ... on each unit that spell affects." So the wording of the Staff itself doesn't care that the spell is a model, and that you are dealing damage based on some models placement. The only things that matter are:
Is it the Hero Phase in which Staff of Focus was used? For our purposes here, yes. Note: this would not add damage to the beginning of the round effect.
Was the Everblazing Comet cast by the unit(bearer) who used the Staff? For our purposes here, yes.
Does the Everblazing Comet spell deal mortal wounds? Yes.
Therefore Staff would increase those mortal wounds by 1.
If the Staff said something like "When the bearer of this artifact deals mortal wounds with a spell" then I could almost understand the confusion, but it's very plain from the actual text that all the Staff cares about is a spell dealing mortal wounds when it goes to increase those mortal wounds.