3rd party artists can't create anything that can be used inside Fortnite, neither inside the v-bucks store nor outside of it. There's no 3rd party "market" at all.
Apple has a market where third parties can market their apps. Apple requires a cut of all purchases made through that store, which alone is not monopolistic. Apple also requires that you sell your iOS app and any iOS in-app purchases only in their marketplace. That's the monopolistic part.
No one can create 3rd party items for Fortnite at all, so there is no monopoly on where 3rd party items for Fortnite are sold.
It's not even illegal to have a monopoly; it's illegal to use your monopoly powers to keep other parties from competing, and that's exactly what Apple has done.