Its insulting because its an arrogant and condescending way of calling out someones low dps. A better approach would have been asking the fellow what rotation he uses and then making a suggestion like - hey I find if I put spell y before x and then follow with q my dps soars, see if it works for you.
When you call someone out you make them defensive, they didnt ask for your help or advice, in wow and in real life if you try your scenario I'm sure you'll find out asap people will get offended more often than not.