The point is that Riot doesn't call them that any more (or, more specifically, they call YASUO that but not any of the others) because they decided that melee ADCs were a bad design choice that was impossible to support and turned them all into assassins or bruisers instead.
So presumably when they say "light fighter" and explicitly don't say "melee ADC" they're trying to draw a distinction, although I guess I could be wrong about what that distinction is.
And again, Riven is the example of a champ that makes "light fighter" work without having to have untargetability, invulnerability, or anything insane. Just a lot of maneuverability and probably a little too much burst.
I buy the inclusion of Trynd as a light fighter, although personally I just think of Trynd as a terrible champion.