Why does Gangplank's Q proc Kleptomancy off of Pantheon's passive?

Pantheon's passive, according to the wiki, should block any basic attack from a champion, large monster or turret. With that he also blocks/negates almost all on-hit effects from basic attacks, with very few exceptions. The wiki literally mentions GP's Q (Parrrley) and Ezreal's Q (Mystic Shot) as abilities with on-hit effects that will be negated.

However, Gangplank's Q will still pop potions off of a Pantheon whose passive is up, which makes 0 sense. If Klepto is going to be considered an on-hit, it should be blocked by Pantheon's passive.

My question is; Why isn't it blocked? Before you brush it off as someone complaining about their main, understand it actually has huge impact on the lane. Before, GP got hard countered by Panth as GP had awful early and Panth cheeses off of that. Now, he can just Q and Panth can't do anything about the free sustain GP gets in the lane unless the GP is just really bad.

I haven't checked, but I'm interested to see if this is also an issue with Nasus/Panth interaction.