"Necrophotic" refers to the Old Gods. Considering Icecrown Citadel is constructed out of Old God blood, using the word again there is not that surprising. Also, if the word was a random creation instead of an intentional choice as you hypothesize, it would be unlikely to be used twice, given its unusual meaning.
Yet as far as we know none of the "old god's powers" manifested themselves inside ICC, other than Yogg-saron's ability to look inside of it. And it could be a case of "recurrent dev memory" where they heard it somewhere and decided to stick it somewhere else... The people that code and name items could be completely separate from the people that write the lore. For example, there was an item that drops from Ossirian the unscarred named "Helm of domination..." I very much doubt it has anything to do with the Lich King's helm of the same name.
But I digress.
"Do not look down, my friend. Even in the darkest of times, there is always hope... Hope for a better day, hope for a new dawn... Or just hope for a good breakfast. You start small, then see what you can get." ~ Covetous Shen
I think the powers of shadow priests come from the same place as the powers of the Light - a place that is the source of many powers, but is native to the universe of Azeroth and all the other worlds.
The Old Gods come from a different universe, an entirely separate realm. They are described as 'Necrophotic' lifeforms by the titans, a made-up latin word that means 'dead light.'
I believe the Old Gods are manifestation of the 'Light' of their universe, a force that is the opposite of the Light of our universe... a kind of 'anti-Light.'
Um you're wrong
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Originally Posted by Wyrt
No, it's not. Even Naaru have a shadow state. The Light doesn't exist without the Shadow and vis versa. Naaru are the embodiment of that having both a light and a shadow state. Nothing to connect it to old gods.