I think the question that is important is not what the technical differences are, but rather:
a. how much can you do, if you do not have access to this ability? Do you have a follow up to after it ends? do you have a setup that you can/need to do before you use it? Is there anything Mr Yi. is good for, outside of alpha strike?
b. how much are you getting done, while being in the untargetable state? Is it a bit of damage and a dodged skill and after that you need to have some good follow up, or have you just won the team fight right there? Are you doing the equivalent of a single AOE skill in damage, or are you fully DPSing whilst being CC-immune?
Technically alpha strike and blade waltz are similar but not identical. Blade waltz procs on-hits and interacts with AA modifiers including defensive ones (yes shen's circle will completely delete blade waltz's effect), alpha strike is pure damage. I think Blade waltz also "waits out" momentary untargetability effects instad of canceling, and it doesn't reset firoa's position if it cancels or kills target.
Fiora has one thing that is a problem in her kit. Her "gameplay fantasy" is me>you. Which is something you can't ever really give regardless of "oh she's supposed to be a duelist".
Old fiora i think lacked distinction. What was she aside from her ult? another Irelia? another Xin? take their ults and put them on the bench, what difference do they have? New fiora has low CD dashes and she has a spell shield so she's not Xin Zhau with a different ult.