AP Gangplank

I am not sure where to post this since it is pretty random. I was interested in trying out AP Gangplank so I farmed up in a custom game. He is pretty much the same as he was before, big heal, big ult, decent Q damage with Lich Bane. The only problem I am seeing is the interaction with his E.

Now the way I am reading it, any damage he does to the barrel with an attack should be what the barrel itself puts out. But it doesn't seem to take into account any of GPs magic damage from Lich Bane etc.

Now, I am not sure why this would be intended, as its not like AP Gangplank would be OP, since his early game doesn't exist. I notice the text saying "the attack's damage" is highlighted in orange, for AD, but I feel like that could mean it does physical damage as it says. If this is to say that it only does the physical portion of what damage he does, shouldn't that be more clearly stated somewhere?

Anywho, just thought maybe someone else knew more about this.