When you give advice that could screw pretty well someone's life, probably you should prove the advice is beneficial or not harmful at least.

It really comes down to phrasing. If they had said "Have you tried a ketogenic diet?" Or "Some people have found a ketogenic diet to be helpful with type 1 diabetes." it would be fine. Instead it came off as an order or command: "Switch to a ketogenic diet."

One of the things about written communication is we only have your phrasing and word choice to inform us of your tone and meaning. It is vitally important to use proper phrasing to avoid being misunderstood. It is entirely possible that their comment was innocuous but it came off as rude.