You can write software for Fuchsia without using Flutter, but Flutter is the most friendly way at present.
In a basic sense, Flutter is based in Dart, but if you want to call into another language like Swift, there is a way to do so, called FIDL. I'm not sure if Swift has FIDL bindings yet, but I'm sure that it and many other languages will, in time.
Even today Flutter has support for Swift code on iOS the same way it supports ObjC, via MethodChannels. (This is also true for Java and Kotlin code on Android.)
In a basic sense, Flutter is based in Dart, but if you want to call into another language like Swift, there is a way to do so, called FIDL. I'm not sure if Swift has FIDL bindings yet, but I'm sure that it and many other languages will, in time.
Even today Flutter has support for Swift code on iOS the same way it supports ObjC, via MethodChannels. (This is also true for Java and Kotlin code on Android.)