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.
The next level up we start to see responses to the writing, rather than the writer. The lowest form of these is to disagree with the author's tone. E.g.
I can't believe the author dismisses intelligent design in such a cavalier fashion.
Though better than attacking the author, this is still a weak form of disagreement. It matters much more whether the author is wrong or right than what his tone is. Especially since tone is so hard to judge. Someone who has a chip on their shoulder about some topic might be offended by a tone that to other readers seemed neutral.
So if the worst thing you can say about something is to criticize its tone, you're not saying much. Is the author flippant, but correct? Better that than grave and wrong. And if the author is incorrect somewhere, say where.
I wasn't disagreeing with the statement about the ketogenic diet, I was disagreeing with this statement:
"Sometimes statements are statements. People needlessly attach emotion to the written word when there is no need."
In my disagreement I explained how the author has an imperative to make sure they are understood and, since we only have the words that they posted to go on, it is important that they are clear in their wording.
Secondly, I am not responding to their tone but the clarity of their statement. It is a completely different argument.
I was just trying to explain how the comment could have been misunderstood. You seemed to put the burden of understanding upon the audience and I felt compelled to elucidate the source of many commentators 'needless emotions'.
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.