Seems like a death re-cap glitch. Those happen.
She probably lunge+auto reset crit + lunge + auto reset crit + hydra active, where each action dealing ~400 damage vs. no armor in a max-build-for-maps-you-can-get-Pillager isn't unusual.
Fiora is much more an assassin than anything else, and end-game she can become a terror like Fizz or Pantheon where she doesn't even need her ult to kill someone, and she can kill them exceptionally fast.
Insta-gibbed fools with her plenty of times to know it can all be over in the blink of an eye.
Usually with
strats for mega wombo-combos.
That said - Ezreal's mystic shot had to get patched because it was critically striking in rare circumstances when it was never intended to critically strike, and similar eyebrow-raising nerfs happen every now and then. There's one with Shen that happens in extremely rare cases.
There's a really small off chance Fiora's lunge could have bugged out and critically struck, so if you have a replay available, test it to rule out Recap failure, and submit it to bug-reports that's a rare possibility.
Was most likely the re-cap messing up with an assassin assassinating though.