Apparently, importing ~/.config/kglobalshortcutsrc is enough for the shortcut to show up in the System Settings, but not enough to load the shortcut. Disabling, applying and re-enabling and applying again also doesn’t help, and neither does re-assigning to the same shortcut, which resulted in this lovely error dialog.
I never saw the Alt key referred to as anything else than Alt
On MacOS it is option or something
Mostly older Linux apps refer to Alt as Meta. But since those apps predate the “Windows key”, it seems foolish to also start calling that meta.