To me, I don't have FYM, despite meeting your threshold 8x over. It would not be easy for me to replace my current income at another job. (I'm 7+ years here, know our tech and business inside and out, and am paid pretty fairly for that, but that doesn't mean that I have nearly the same value to another company, something that headhunters who call have a hard time grasping when I tell them my comp range.)
It also doesn't have to be your employer: it can be anyone you're financially tied to in some way.
I can't give a comprehensive survey of everyone who's made FU money, but take Tim Ferris for example. He's not a whiz at building websites: he just knew to cash in on selling brain supplements online when that stuff was taking off.
Edit: I should point out that I don't think FU money is exclusively about being able to say "FU" to a jerk. I emphasized that aspect just to invert the comment I was responding to. FU money can also just be about ditching a career you're not that interested in (but may be highly skilled at, and otherwise locked into).