The default_external_links_in_new_tab setting was added relatively recently. Before that, only end users could change this via their user preferences.
I am guessing the issue is that when that setting was added, it was implemented such that when a new user is created, their initial preference is set.
But the logic still holds that the setting is only effective for the current user via their preferences.
This may or may not have been a conscious decision, so I think you should recategorize this topic as a “feature” request.