What is this a quote from? One reason that it can make sense to say that Herbalist is Potion-related is that one of the defining characteristics of the Potion-cost cards is that often you can only get one per shuffle. Sure you can buy more Potions, but that sucks for other reasons. So Herbalist allows you to get more than one Potion-cost cards in a shuffle. That's important.
I'm more interested in why it says that Possession is not useful in multiples!