
This ‘Preferences’ menu, which can be found in the Keymap tab seems to be only available with the default Blender keymap?
If you switch to a different Keymap profile, this menu disappears. and the settings you set here are ignored.
I believe in older versions of 2.8 this menu used to be in the Input tab.
Interestingly any profiles that were made/derived from the default Blender keymap will respect the settings you set, but the menu just isn’t visible