I do think we're going from a lot of different, individual definitions of JFE, and of flattering itself.
The definition I've had in mind for "conventionally flattering" is outfits that echo the supposed ideal female body. So, things that make one look tall-ish, thin, and narrow-waisted, with some sort of ideal proportion between neck, torso, and limbs. A lot of us have grown up with the idea that the right clothes are ones that play up our features that match these supposed ideals and de-emphasize features that don't. For instance, someone with my shape might prefer heels with a skirt, or cropped jackets to make short legs look longer, or tops that come in at the waist rather than hang straight down from relatively wide shoulders. Or colors that make pale skin look more alive.
I've enjoyed learning which of those concepts work well for me. My own perception of my proportions, vs what someone else would see, have required a lot of adjustment. I just bought a jacket that is, alas, way boxier on me than I realized, in a way that just isn't the look I was going for. So understanding figure flattery is helpful.
But a lot of us also reject the idea that each outfit should cater to that goal of the ideal figure, and that's the definition of the JFE concept that I use. Heels might look better with my short legs, but I think it's just fine to wear flats that make my legs look shorter. A top that fits well in the shoulders but makes me look two sizes larger is often exactly the look I'm going for.
It's a separate concept from whether I love something or feel fab in it. Angie's original post, to me, suggested JFE as an intentional style component, not a compromise. So for me, colors we dislike, or summer clothes that make us feel meh, don't fall under JFE so much as the occasional necessary compromise.