It's GTK on Linux, win32 on Windows, and Cocoa on macOS.
Is it still a wrapper over wxwidgets?
No, it binds to the frameworks I mentioned above directly.
How one defines "native GUI" is very simple: it uses the native widgets of the desktop OS, rather than custom-painting its own onto a pixel-level canvas. Buttons, input fields, lists/combos, context menus, the file picker dialog, what have you, are from the host system.
it's not that simple on windows, at least: https://raphlinus.github.io/rust/gui/2022/07/15/next-dozen-g...
I'm always a bit skeptical of these statements. How does it define "native"? Does it use gtk/libadwaita on Gnome and Qt on KDE? What about windows, does it use Win32, WPF, WinUI2 or WinUI3, all of which could be considered "native"? What about platform-dependent layouts?