We have an affix on item that says "Your X spells deal Y% more damage", with X being the damage type, i.e. Fire, Physical, Poison, etc.
If you look on spell runes, some have a little adnotation that says "Damage of Y Spell changes to X", with X being the very same damage type as above.
We have CoE which cycles through damage types.
^This is exactly what the information about Necro "poison skills" refers to, not a general theme of an ability/rune.
To make things easier to understand, if we had like a simple Fireball spell, it's theme would obviously be Fire, but for gameplay reasons they could set the damage type to whatever they liked, for example Cold. Now, a Fireball that would look like fire, but deal cold damage wouldn't make much sense, but the difference between Poison damage type and Blight/Decay theme is small enough to put them on the same spell. And that's what Blizzard is doing.
So there are necro poison skills/runes or not? I dont get it.