It's not, because it's remarkably hard to find someone who is a "dark twin". People's life-trajectories are remarkably different, being chaotic processes, so it ends up being counter-productive to compare yourself to someone who seems similar to you. In general, your belief of how similar they are to you, or how aligned your goals are, is just that. A belief. One that is extremely difficult to prove, and might have terrible consequences (e.g. depression) if you're wrong.
Let me put it this way: even actual twins, who grow up in the same household, are usually not worth comparing between. Contrary to popular belief, twin-studies are not well-accepted as good science, in part because we are beginning to appreciate the complexity that is gene regulation. In particular: epigenetics.
(Humans don't have the largest genomes, but we do have genetic control systems that are far more sophisticated than those relatively simple organisms with larger genomes. More generally, mammals have sophisticated genetic control systems, compared to other organisms, but even within mammals, humans beat rats.)
I think you are taking the "dark twin" phrase too far. There were plenty of differences between Tesla and Edison, Musk and Bezos, mentioned as examples in the article. The concept of nemesis isn't built upon conscious beliefs, but "repressed attraction" as the article states. There's nothing fundamental about this concept that requires that you be similar in every way to the nemesis. A few similarities in the right areas can be enough to drive the repressed attraction.
