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.
So I interpret this position as simply the most difficult hand position to maintain, thus indicating some intention, practice, or awareness -- and thus self-control, which was considered the master virtue classically.