Relatedly, Harry claims to be the only wizard in the Chicago phone book. But in any sizeable city in our world, you’ll find a fair number of people making their living selling magical services. I know of multiple shops in Cleveland where one can purchase spells, and there are probably more I don’t know of, since I’ve never even actually tried looking for them.
Now, presumably, those spells don’t actually work. And presumably, there are people in Harry’s world also who claim to have magic powers but don’t. So maybe he just means that he’s the only real wizard making a living that way. But that’s not what it sounds like he means.