That's a poor and unnecessary assumption. Just in the last handful of years there has been a proliferation of good quality bodycam footage that gives unprecedented insight into police encounters. It would be more charitable to ask where the commenter's perspective is coming from.
"Situations where police are able to survive only because they were able to quickdraw and shoot a criminal first like some sort of cowboy are vanishingly rare and possibly purely fantasy."
It sounds like you may be the one who needs some experience watching body cam footage and reading about incidents. Just off the top, here is a quick refutation of your "fantasy" statement (and there are many more like it):