It means you're making an assumption, as there has be no official statement about this skill's status.
It could very easily be bugged, however, that can mean two things.
First, the description could be wrong and it isn't actually a marksmanship skill. This is most likely.
Second, it could be a ranger skill, but it is just bugged and so isn't available to players. The problem with this theory however, is it would mean that rangers would get 1 extra EotN skill. Except, there is the fact that warriors have their own extra armour set, making an extra ranger skill somewhat probable.
and youre also making an assumption that its a player skill.
before the expansion came out, the skills available to each proffession were added to the wiki, how many of them where not them?
i dont see why the description is wrong, its a monster skill tied to markmanship, why is it tied to it? perhaps the creator of the monster (and maybe the skill) was intending to make all the ranger destroyers to have it, but they (as there are two class of ranger's destroyers and the two of them could have varying ammounts of attribute points placed on the markmanship attribute) changed their minds and decided to make it that only one has it, and prefferred not to change the skill wording-function, but removed it from the other class.
why it has its own skill icon instead of the generic "monster skill"? maybe the creator got a little bit of extra time to mess around with it

or perhaps he did the skill before making the destroyers.
also, the destroyers of horde come in lvl 15 (mentioned in the wiki, maybe from the challenge to defend the hatchling dragon?) it would be "interesting" that, if the skill was placed as a monster only skill, and the lvl 15 had access to it, they would deal the same damage as those from lvl 28, as most monster skills deal the same damage/function regardless of lvl and/or class; this would be a first of its class, ie having it linked to an attribute.
as it stands now, its a monster only skill. when and IF they say its bugged and decide to add it (making 151 skills instead of 150) then it will become a player's skill.