If a judge allows a proof, it is typically not considered an error... but a judgment call.
However, errors are why there is an appeals process. The appeal found no error or, to your point, that the error made did not materially effect the outcome.
You will find that the introduction and exclusion of certain types of fact is one of the things you have to plan for in court. Court is made up of people and people do crazy things.
Did you know of the contents of the package? If you did, then I see the appellant ruling as proper.
I will say, however, that this is why the services of qualified legal counsel is a great idea....

