These blanket statements are far too broad in my opinion.
In some cultures, that hand gesture is used as a means to ward off
Not to signify the user as a devotee to Satan.
Go up to a country peasant in the South of Italy, and compliment him or her and they will make the same hand gesture as a means to ward off any evil intent or jealous thoughts you may secretly be harbouring.
We have only so many fingers, and only so many ways of contorting them.
It's very likely similar gestures will be used by different people to mean different things.
The pointing of the index finger in an upward gesture in North America is a gesticulation used to emphasize a point. in India, it could get you slapped. in India, it's the equivalent of giving someone the North American middle finger.
Unless you're deaf, ...a Mason, ...or a traffic cop, ...hand gestures are meaningless. ...especially the ones inadvertently made when the hands are at rest like in the previous picture of the US first lady.
That said, ...that's not to say that there aren't those who do use hand gestures to identify themselves & others.