I think the biggest difference between reading a book and listening to an audiobook is the loss of control you have while listening. The words continue to come at you while listening without you be able to control the action. You can't (easily) re-listen to a word that you are confused about and if something attracts your attention while listening you will definitely miss some content.
Sure, you can stop and rewind, etc. but very few do that and if you had to do it much at all you wouldn't enjoy the listening session. While reading text, it is a trivial operation to reread something and even looking up a word in the dictionary is pretty simple.
If you can concentrate completely during the listening session then I feel that there is no difference between listening and reading text; but for most of us the listening session can be easily affected by real life and thus comprehension is diminished.
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