Role based visibility is applicable to navigation items that are impacted by user role configuration. E.g. if your navigation item shows a page, the visibility of the navigation item will be based on the Module Role visibility setting for that page. This is applicable to items that have their own security configuration such as Show page, Call Microflow and Call Nanoflow.
The sign-out function is not part of the security model (perhaps unsurprisingly, as any user might need to sign out) and can therefore not directly be configured based on user roles.
If you still have a specific need to limit the user roles that can use the signout function, you could instead create a button on the Layout you are using for your app. Separating the sign-out into a dedicated button will allow you to set which user roles are allowed to see it.
Do keep in mind that in order to be able to select roles, your Layout has to be in a module that has module roles (the Atlas_UI_Resources module that contains the default Layouts has no module roles). The easiest way would be to duplicate the Layout, move it to the module were you will be using it and configure the necessary there. This will also allow you to update the Atlas_UI_Resources module to a newer version in the future without losing your customizations.
Hope this helps.