we only need Xcode to be installed on macOS since it bundles the iOS SDK (ie all the header files.) similarly, we ask the user to supply a copy of Xcode.xip during the setup process on Linux in order to extract the SDK.
it definitely depends on your definition of “replacement” in the end, but I (and most people I’ve spoken with in the iOS community) would consider “Xcode” to be the Xcode build system, user interface, and proprietary tooling. xtool doesn’t rely on any of this, not even for signing and installation. you can (read: must on Linux, may on macOS) use it with the open-source Swift and Clang toolchain + LLVM’s LLD linker and MachO tooling. Codesigning uses zsign, an MIT-licensed x-platform codesign alternative, and installation relies on the open source libimobiledevice project, which is installed by default on many Linux distributions (eg Ubuntu).