Actually assassins deal damage primarily through abilities, which gives them a burst pattern, whereas fighters have a sustained damage pattern that consists almost entirely of autoattacks and are tankier in order to do so. Plus they usually have cc in their kit in order to lend utility to their team.
Live Fiora doesn't fit either of those categories due to her squishy zero-cc nature and lack of real damaging abilities. Her ult, while it does deal ok damage, is mostly useful for the untargetability and lifesteal, not the damage itself. Also, she has no inherent tank stats or abilities, nor does she typically build any. To further illustrate the difference between Fiora and a "fighter", fighters don't typically build crit. I consider Irelia to be the quintessential example of what a fighter looks like.
Yasuo is an odd champion who plays essentially like an assassin as well, but once again, since he is an inherently squishy champion, I would contend that Yasuo, Fiora, and Tryndamere all occupy the same space; that being the Melee ADC, which has traditionally never really fit into League's meta. Yasuo works because of his extreme mobility and ability to one-combo enemies, like an assassin.
What irks me the most is not only are they taking that away from her because of crybabies who can't deal with aggressive early lanes, but they are completely missing out on the chance to make her a fun, mechanically complex champion in favor of just dumping in easy true damage stats and a thematically out of place AoE heal. Which Janna is definitely better for anyway. Calling it now, you're going to see SO much tank fiora where people just build tank stats and cdr for the constant cc and true damage.
The direction they were talking about taking her months ago was SO much better, in every conceivable way. I'm willing to bet they gave up on it because it was too hard for those slackers to code.