These gestures aren’t exclusive to any particular group—part of the reason they last is that they are easy to discover multiple times and use in multiple ways, plus they seem somewhat natural. It’s similar to how the “devil horns” gesture can also be seen as an owl, a longhorn, or an I Love You. They’re multi-purpose, which is part of why they end up being commonly used.

I’m not suggesting that everyone who uses the sign of the fig is using it in a Masonic or occult way, but rather that some politicians probably picked it up from that background and began using it, and then it was copied by other politicians who liked their style. Eventually, it just becomes a standard political gesture, and only some politicians would be aware of its roots.

It’s possible that they just came up with it independently and copied each other, but if that were the case, I’d expect to see more closed fists with the thumb around the fingers mixed in rather than them synchronizing to the thumb on top (the “fishing reel” as described in this post). This is just my theory—I never had the chance to ask Bill Clinton where his thumb gesture came from, but it aligns with what we know about the connections between American politics and Masonry.

Edit: Here is a good photo of Clinton with his thumb on the middle finger. He did it more commonly in his earlier years, later changing it to the thumb on top with his pointer knuckle jutting out as a base:

https://media.gettyimages.com/id/50372601/photo/bill-clinton-and-hillary-rodham-clinton.jpg?s=1024x1024&w=gi&k=20&c=i2AwLQrnLVW64MFRCNF1_fRJN2D_gNeVo5PE-7L_sFM%3D