Amongst many other reasons, my biggest is it’s not searchable by search engines.
Well gee, I hope you don’t use texting, phone calls, emails, private forums, social media DMs, or talk to anyone IRL, because those aren’t searchable either!
This argument seems like reaching for something to complain about rather than having a legitimate problem with discord. If anything, you don’t like the “large group chat” paradigm, but that’s like hating a screwdriver because it’s not a hammer.
Pretty sure I would quit on the spot. Clearly doesn’t understand “clean” code, nor how people are going to interface with code, or git for that matter. Even if you write a book for each commit, that would be so hard to track down relevant info.