Yes, if you are the one fishing out the rare you will get a 100% chance for the mount it drops (I got mine).
But this beautiful Aurelid, drops a few more goodies, all of which are locked to specific classes and specs to look thanks to the wonders of personal loot. Like that 1 handed liquid-light-saber.
So I believe that at some point the players need to be able to get extra aurelid lures from Vashj or this mob could turn to be a really frustrating one down the road once most people got their mounts and are done with fishing up this mob.
Yeah that one makes no sense to me either, it’s a chore to get the lure already , why not let folks fish it up once a day/week or sth like that for instance
I actually brought up this very same point on the forums earlier today.
If (for whatever reasons) you don’t get the mount, you can ask Lady Vashj for another Lure the following week (according to WoWhead). But once you get the mount, you can’t get any more Lures from Lady Vashj (it seems).
Luckily for me though, I happened to be in the area today when someone pulled the Rare, and I got the Sword as a drop. So it’s not an issue for me anymore.
However, I do agree this will become an issue for players in the future who just want the Rare achievement.