I had a friend in undergrad that couldn’t eat in the dorm cafeteria because everyone around was chewing and chewing and slurping and just eating normally but he found it intolerable. He thought it was the grossest thing possible - so many people eating in public.
He came from a farm in rural farm town. He probably never had been around more than a few people at once, let alone eating together.
Anyway in a few months he was normal and had no problem eating in a crowd.
If it was intolerable due to cultural differences or him perceiving it as rude, that's a different thing than misophonia.
Misophonia is when you have an uncontrollable fight or flight reaction to trigger sounds. You can think of it as a miswired nervous system that reacts to certain sounds with fear and/or anger. In this case it's not about preference ("I find the sound rude") but something deeper and more neurological.
It sounds like your friend probably didn't have misophonia if he was simply able to get used to it after a few months.
It's deeper, but can also appear/disappear and have a cultural component.
My closest mental image is people genuinely enjoying some soup, to immediately puke it out as they hear it's made from crushed bugs, even as it's a lie. The reaction is visceral and uncontrollable, but the cause is not physical and is deeply rooted into their mind.
It sounds like what you describe. He was absolutely disgusted by people eating in public. And he described it as the chewing and swallowing and noises that he couldn’t stand.
I made it a point of going to the cafeteria with him and my friends would join us - usually in a table of 10, mixed sex. In retrospect it was probably torture for him. At the beginning is when he confided with me about his phobia about eating but maybe out of naïveté I didn’t care - he was my friend and he ate with us and eventually it didn’t seem to be the big deal it was when I first met him, especially after he got a bf in the friend group.
He came from a farm in rural farm town. He probably never had been around more than a few people at once, let alone eating together.
Anyway in a few months he was normal and had no problem eating in a crowd.