Duel someone. The check for buffs are there; You just cant see them by any normal means. Just because you cant see invis, doesnt mean a mob isnt there, you just have have the right tool on how to look.
I'm not sure who is right off hand in this discussion, without getting online. However, I can say that this logic is not always true.
ISXVG is only capable of accessing information that your VG client "knows". For example, VG could be coded in such a way in that if a mob was invisible, your client may not know it's there at all. That means that NO TOOL ON EARTH will be able to tell you about that mob. Again, I can't remember off hand how VG does it, I'm just saying that this logic, while making sense, should not be taken as truth.
I can tell you that there are a LOT of things in VG that are sent to you directly by the server and there's nothing I could do to help you with it. For example, ISXVG can't tell you if a spell is going to cast successfully or not. The way it works is that you cast a spell, the client sends a message to the server asking if the spell "can" land...and then the server sends back a message saying that it "fails" or that if it doesn't ...THEN things happen.
That's just an example; however, the best thing to think of when you're thinking of what's possible with ISXVG (or any modern game) is that the client probably will not know anything unless it *needs* to know it. So -- does the client need to know a player's buffs when you're dueling them? Perhaps ...but, when you're dueling...doesn't a player show up as your offenseive target, not your defensive target? I was under the impression (though I haven't played for a while) that buffs were available for offensive targets (PC/NPC/etc) and 'encounter' members ...but, were they available for group/raid members or any 'defensive' targets?