Is there not a FOSS store on Android that provides app signing and doesn't charge anything? This is an opportunity for Fortnite's creators to create their own store which then installs their app.
This is kind of what they did, "Epic Launcher" installs it and keeps it up to date. It seems to be something like steam/origin/etc but for android
Such a store would either require that the developer pay for payment processing and hosting themselves, or would have to charge in order to cover those costs.