An Apple Developer account is $99 a year and the only prerequisite to putting an app on the store. If it’s free, there are no other fees. I wouldn’t call $99 a small fortune.
There are many “open source” apps on the App Store, though most may argue they technically are not because you never have the option of compiling yourself, so perhaps “source available” would be more apt. Things like KDE-Connect are on the App Store so clearly there is some demand for iOS counterparts to open source multiplatform applications.
Bold of you to assume Trump knows how to write and the letter was at all legible.