Not a theory, thats what it is. But there are caveats with the "new tile" thing. It's probably not really the tiles the map, but the enemies spawning there. I guess technically it's impossible to know if spawning the enemy is what causes the herald, or if one can show up on what otherwise would be an empty tile. I personally think its probably spawning with champions/uniques to be consistent with step#1.
Also important to note that its not new tiles to you, but new tiles to the game. Some jackass can teleport through and uncover an entire zone without ire, spawn everything, kill nothing, and essentially brick heralds from being able to spawn there once ire is drawn.
Yup, saw that in multiple TZ shard games affected due to this. Especially at river of flame due to Pokémon hunting, key farming areas, cs etc.
The spawning mechanism seems to have changed from pre rotw. I may have remembered wrongly, but I could recall passing by an area eg. halls of pain to do nith, then when it terrorises later, the initial mob will also be terrorised if u left them for some time.
Now, they just spawn and remain as non TZ.