Actually this isn't necessarily a clear cut mistake which is why there was a 4:3 decision on the issue. Its quite complex and the judges original decision may well have been based on some complex considerations. I offer no opinion as to if he was right or wrong but 3 other judges also agreed with him so on balance that 4 out of 8 judges reached the same conclusion.
This issue comes down to whether the common law rule of admitting prior bad act evidence for non-propensity purposes applies. Normally the answer is no so prior victims cannot be heard to help prove that he did it before.
However there have been changes to the law in many states in recent years to provide an exception to this rule for cases involving acts of intimate violence which sexual assault is. I know that the NY changes were proposed in 2019 however I'm not entirely sure what the exact wording implemented was. Some states chose to link the definition to acts against an "intimate partner".
I suspect that the debate here was whether either victims met the definition of "intimate partner" since neither appear to be willing partners. Weinstein may have got out on a technicality.
