Is it really unnatural? Interestingly, as a right-handed, the two middle fingers of my right hand tend to effortlessly group together; this feels noticeably less true on my left hand, but still observable if I try to relax it.
The peculiar "mission tile" (half-cylindrical) flexibility of the palm region, encouraged by writing for example, may foster this grouping.
It's a bit surprising for the article not to address potential anatomical causes.
Your example of writing makes sense too. Perhaps because writers were a more exclusive club then, prominently showing your subject’s hand with that finger grouping was a message in itself.