Why is a vegan diet unhealthy for cats?

Hello!!
I am a vegan who was actually arguing with a fellow vegan as to why cats shouldn’t go plant based because of the lack of consistent information and (unbiased) studies on it. Also included links to sources relaying the possible health risks and issues that come with a plant based diet. I’m also aware that cats on a plant based diet must be monitored super closely and have frequent blood tests. The other vegan user claims that their cat’s blood levels were PERFECT and that they are completely healthy.
Good for them… but I also have read that vegan cats are a case by case study and not necessarily recommended. They got pretty aggressive with me and even reported me to try and get me banned from the subreddit for advocating against giving cats a vegan diet, despite claiming to be a vegan myself. They told me I wasn’t a real vegan.
Anyways, can any veterinarians or technicians tell me why else cats going on a plant based diet isn’t recommended and even bad for them. Feel free to comment any articles and studies below. The articles I sent just were not enough for them to handle. In fact, here is a paper that they themselves wrote about feeding cats a plant based diet. Mind you, this person is in no way a specialist or has the credentials to even back these claims.

https://docs.google.com/document/d/1SWKO\_jjuXu28vND5cdSYIBFZdZXDwmnWuJv9HjvuYqU/mobilebasic