That's just untrue on the face of it. All of the build tools are open and cross-platform. Is there a specific piece of Android Studio that you require for Android app development?
Not certain if this answers the question, but it seemed like you're generally expected to install Android Studio to get the correct build versions of all of the tools and libraries. I guess theoretically you could repackage them yourself, but also not entirely clear why you would—other than perhaps download size. The tools can be driven externally, once installed, but so could XCode projects (with `xcodebuild`).
Not certain if this answers the question, but it seemed like you're generally expected to install Android Studio to get the correct build versions of all of the tools and libraries. I guess theoretically you could repackage them yourself, but also not entirely clear why you would—other than perhaps download size. The tools can be driven externally, once installed, but so could XCode projects (with `xcodebuild`).