It's actually native now, albeit kind of a mess (or at least the C++ version is; I haven't used Delphi). For example, Windows/x86, Windows/x64, and ARM are all different compilers (OS X and ARM are both clang; x64 is an older clang, x86 is totally different) that support different features and have subtly different behavior sometimes. Then there are differences between VCL and Firemonkey, so don't even try porting an existing Win32 app.

It's only portable if you try really hard.