Mm'kay, interesting. Although I will clarify that Fionn's F/GO profile references him fighting Aillen and Nuada as two separate instances over the course of his life so it doesn't appear that they are confusing the two at the least.
I have not found a source referencing Fionn ever battling Nuadu, but I'm also open to being wrong - I am not as strong in the Finn Cycle as I am in the Ulster and Mythological Cycle.

(One day I will finish this stupid document for a Lostbelt based on medieval Ireland.)