How in the world does a test of 'does this colored text match its name' test 'self-control'. 'Self-control' as used in the article means things like 'can you stick to a diet', or 'can you keep your word', not can you control your thought processes enough to quickly name a color. You can be highly self disciplined and bad at this task, or highly undisciplined and good at it. In fact, I'd imagine undisciplined people might be inspired by the entertainment value of this 'game', and spend useless time practicing it rather than doing what actually matters.
You have to stay on task (say the color not the word) and this requires a bit of working memory (what was the task?) and requires inhibition of the 'fast system' (system 1, which would just blurt out something), all associated with the PFC (prefrontal cortex).
Obviously it's not perfect, but at least very simple, quantitative, cheap, etc.
Staying calm in stressful situations is very powerful.
Power is about managing conflicts, being able to control yourself while going through one is a huge advantage. Being able to evaluate your options, using the right amount of time for decision making, thinking through what you are actually going to say before starting to speak, these are all skills that depend on self-control.
Yes, it's not 1:1 power, but as the submission title states, it signals it. (Hence the movie cliches about extremely cool guys, don't even flinch while an explosion is going on, and the memes about how millenials flinch when the phone rings, etc.)
This study is complete garbage.