there is no gain to thigh roll the body of the string cause you have to separate the string again
Correct - that's why you don't thigh roll the body of the string
I'm talking purely about thigh-rolling INSTEAD of the slower "twist, wrap, twist wrap" thing. I'm not talking about a different way to make a string, I'm talking about a different method to make EXACTLY the same string.
Basically - if you took my string, and put it next to a Flemish laid in double looped string made the slow way, you wouldn't be able to tell the difference - in quality, tension, method etc.