Yes, this is myth and folklore heard throughout the vacuums and echo chambers of hunting forums, but 100% false. There isn't an ungulate in N America that clan stop a bullet on their shoulder. The statement is obsurd.
Dad told me to use this, so it's true. I read this online so it's true. 2nd favorite: Guide said this so it must be true. My favorite: my gunsmith told me this, so it must be true.
If you think of it in terms of annual sample size, the average hunter's sample size is less than one. Guides are worse because they will regulate and prohibit less than what they recommended on their hunts, so they never see anything different than continuous high-miss percentages with their clients and their magnum .30 and .338 calibers they can't shoot accurately due to the recoil and added stressors of the hunt outside of the bench at the range.
Hundreds of elk and deer are culled with 6mm ammunition annually, so we get an accurate sample size of actual terminal performance if we choose to watch, look and listen to that metric.