iOS is only a target (and closed) os, mostly people use other OS (like Mac OSX) to develop app for iOS. You can't even copy a .so file onto your iOS device. Who will run lua script manually and load .so modules on iOS?
Some people write stuff just for themselves and never submit it to Apple, some people run private app stores for their companies (which is allowed by Apple).
However, the vast majority of times someone is making an iOS app, it's to submit to the store. I would suggest the default should be to turn LUA_USE_DLOPEN off on iOS.