When we read medieval discussions on deafness, it's easy to judge. We are so used to knowing that deaf people are intelligent, rational human beings. We ask How could they have been so prejudiced?! How could they not realise people who can't hear are still people?!
Now imagine you're in a shop. In walks a tiny little Chinese woman (or a Filipino man, or a pair of Argentinian teenagers) and in broken English she asks you how much an item costs. You have a difficult time getting the answer through to her, and then she asks if you can hold it for her - but all you hear is accent and mumble. Your routine of ringing up items and moving the line along is broken. Be honest - what goes through your head?
If you're normal, you probably get kind of frustrated. And you wonder why she doesn't learn better English. You wish she'd just go away so you can handle the next customer. This person clearly wants something, but you have no idea what. You forget that back home, she was a trauma surgeon. How could she be, when she can't even make herself clear in a shop?
We all do it. All of us. It's a rare person who can endure a frustrating five minutes of garbled non-communication and at the end still have no pre-conceptions of the other person at all. Intellectually you know that the other person is a normal human being,* but the failure to communicate is so limiting that you forget.
Now imagine someone who cannot speak at all. Someone who has never spoken, and cannot hear you. Communication is made by awkward, goofy body language. To the best of your knowledge, no such person has ever learned language of any kind.
We take communication for granted these days. We have so many gadgets and resources, and we're used to the concept of sign language, at the least. Everyone can read and write, in a pinch. It's easy to forget the time when we had none of these modes of communication.
* And that if the situation was reversed, you'd look a right doofus, too. Probably worse.