Never heard that before.
Now if you're shooting targets.. Yeah I get that. 10x is all you need for shooting 1k yards because at 1k your target is the size of a car, not moving, and not in some random shaded angle.
20X max. if the scope happens to be a 3x- or 4x- at the start
Nope, less is more when shooting from field positions. If you can always get prone and your rifle never moves and the animal drops in place, sure, run the scope up to 20x.
Typically, guys get in trouble using too much mag. They want to pick out the hair on the elk they wanna hit. They pull the trigger, and there's a big event that occurs and they lose the elk. It's only moved 20 feet right, but because you're zoomed in on 20x for a 750 yard shot, you've lost him. You have to turn the mag down and then back up in order to find him. If they were prone, sure maybe they could've stayed on him. But it's usually pretty rare to find a place where you can easily get prone. In fact, if I think back from all my kills the past few years, I can't think of one instance where I (or anybody I hunt with) even had the option to go prone. The minute you get off your belly, all bets are off the table, and a wide FOV and modest magnification will gain you more than anything. When you're rested on shooting sticks in a seated position and the gun goes off, you move off target the same amount as when you were on 20x, but your FOV allows you to not lose the elk, but see that he's moved down and right that 20 feet because you're not zoomed in on hair number 5,486 on his left shoulder.
Just run a scope on a modest magnification, and realize you don't need to shoot "1/4 MOA all day long when you do your part" to hit an 18" kill zone on an elk. You're putting holes in vital organs, not shooting cloverleaves on the shoulder. If you can hit an 18" plate at a distance, that's all that matters for a practical elk hunting application. If it's mule deer, bump that down to 12"
If we were talking coues deer, then it's probably a little different, but I won't opine there since I've never hunted coues.