Thanks for clarifying this. I wasn’t being sarcastic – I genuinely believed that you are usually polite and helpful, and now believe that you always are 
I misunderstood your response and apologise for that.
Yes, I’ve followed the current setup instructions which don’t create a non-root user. I have a feeling that earlier instructions did but can’t remember for sure. I’ll ignore the error.
Some sort of wording that makes it clear that the warning is from or endorsed by the Discourse team, and that it must be followed, would have had the effect you wondered about.
An explanation within the warning/advice would help, as proposed above, since on this occasion it’s not the case that it must always be followed.