I selected the setting default_external_links_in_new_tab in the admin area. This resulted in the expected behaviour of setting default preference of all new users set to ‘open external links in new tab’.
However, for non-logged-in visitors, the behaviour is still the old one, it is opening the external links in the same tab leading visitors to go away from the website. When admin selects the default_external_links_in_new_tab setting, it should change the behaviour for non-logged-in visitors as well.
The ‘false’ behavior was from acting on an undefined value returned from Discourse.User.currentProp() when there is no current user, so it seemed to me to not be intentional one way or the other.