Some things are easy to check, like the fact that the Z80 came out in 1976 when Woz was already finishing the Apple II so he couldn't have considered using it for the Apple I.
I haven't personally looked into whether he considered using the Z80, but your statement there is oversimplified. Define "came out". If you mean "announced in public" or "officially went on sale", then that doesn't prove anything. It is quite plausible that anyone in/around Silicon Valley and/or in the Homebrew Computer Club knew about the Z80 before it officially "came out".
But, yes, history IS hard for exactly these kinds of reasons. Too many people are concerned with "first" and "invent", etc., when what really matters is generations of tech and overall impact.
