Man, I had to HUNT for that "Wizards" reference comic. I had the general right place in the archives, but I forgot that Mr. Verres delivered that bit of exposition as a sort of bonus trivia. I kept going back and back through comics earlier in Sister II
when I was actually just a couple comics away in the opposite direction.
Anyway, the point is that being a "wizard" in EGS means you can learn other people's spells. Anyone can get magic, but they're generally stuck with whatever spells they happen to get. Wizards aren't so limited, making them potentially a LOT more dangerous.
Of course, Diane and Charlotte aren't using "wizard" in that way. They're just referring to some dude with magic.