I think that logic is just as faulty as the assumption that 90% of the un-appealed claims would also be overturned.
I suspect many people just don't know that they can appeal. Those that do might think it's too difficult to do so, or believe it requires some specialized knowledge to do properly.
And this is a perfect example of the type of conversation that happens when the correct answer is that we don't know the answer but everyone keeps talking in circles pretending there's a way to know with any certainty other than testing all (or a carefully chosen random sample) of the other denials and getting the actual data. Whenever there's a disagreement where both sides seem reasonable it means that both sides are wrong because the correct answer is that the information present is inadequate to distinguish. All the potential reasons for things going one way or another are also just hypothesis to test since the gut feeling could be right and the reason wrong, and just getting a percentage on the rest isn't enough to figure out why that percentage is what it is
I suspect many people just don't know that they can appeal. Those that do might think it's too difficult to do so, or believe it requires some specialized knowledge to do properly.