Zurai makes a good point in this thread, in my opinion. Just because you can wrangle some kind of, "Well, as a DM I'd never let a player get away with this!" thing doesn't invalidate the fact that the rules are unclear or need to be changed.
Players can pick this item up at level 23 as well, so it's not like it's a massively expensive or "only at level 30" item.
To be honest, I think I'd like to see the cloak errataed to be the same as other invisibility - if you attack, you become visible. In my opinion, invisibility is best balanced for scouting, surprise attacks, and retreat.
What I think happened here is that the writers simply forgot to make the same stipulations about invisibility on the cloak as they did in the other occurences of the power.