When you say “You don’t know that”, you expect the people reading your comment to interpret it generously. A good interpretation of your comment is something like, “You’ve provided no reasoning to back up that argument” or “I think it’s unlikely that you have evidence to support your claims”. A bad interpretation of your comment is, “I can answer with certainty whether you have this specific piece of knowledge, and the answer is no.”

I encourage you to apply the same generosity to comments you read.