I was doing some research on different aspect rations and black bars vs stretched because I'm really undecided what I want to stick with. And I came across this comment by /u/Lucky_Kvack.

It feels the same its just your mind messing with you because your eyes are lying. Then that's a special case. Most people aim using in game objects and not their representation on the screen. For instance you aim from short to long, not 3cm on your monitor. If you aim with your monitor screen then you're hurting yourself. Classic examples why aiming in game is better than monitor screen aiming. 1. flashbangs mess up your ability to see the screen and relying on the screen to aim prevents you from having an easy shot. 2. Changing resolutions will not affect your ability to aim. 3. Changing aspect ratio will not affect your ability to aim. 4. Changing monitor will not affect your ability to aim. If you ever get a new monitor, old one breaks or you go to a LAN you'll be handicapped. 5. It helps you build a better mental picture of the maps/layouts and situations.

Can someone expand on this point and go into detail about what he means by this and how can I encourage myself to aim properly using in game objects instead of my monitor?