That is in large part because there was no uncanny valley, so to speak. Where it was different, it was different with a purpose and was intentional about it. Where there was no purpose, like in secondary menus, it still used native widgets like users expected.

The parent is talking about toolkits like Swing where things looked[1] almost, sort of, but not quite like the native system. That wanted to be native, but for technical reasons fell short. These are what many considered to be completely unfit for use. Whereas today, designers wouldn't think twice about applying the same kind of almost-native-but-not-quite theme to match their arbitrary whims and think they are doing the world a service by doing so, the UI conventions (to the extent that there remains any) of the host system be damned.

[1] https://upload.wikimedia.org/wikipedia/commons/c/cc/Gui-widg...