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.
It takes a tiny bit of intent to splay my fingers out.
Portrait artists constantly study people very closely and are likely attuned to thousands of little nuanced details that go unnoticed by most people. This is probably one.
Now I’m going to start looking at people’s hands to check their finger positions. That might have been a good exercise for the authors of the paper to determine if this is a natural hand position.