Considering how many countless elk and one moose and countless deer I've taken with my .308's over the last 15 or so years, I'd say that is a complete crock-o-crap probably told to you by a gunstore salesmen, because he wanted you to buy a .300 WM or something else he had overstocked.
I think we may have covered this in regards to pistols, but shot placement is everything, even at 200 yards. If someone cannot reliably put a bullet where they want it to go at 200 yards, the shot shouldn't be taken. I've made shots at 250 yards, but I won't shoot outside that range. Can I hit at longer ranges? Sure, but I can't be positive I'd take an elk in one shot at anything outside of 250, so I don't shoot, I stalk, get close enough and BANG! It's elk steak dinner time.
Don't worry about the Bison, they pretty much drive you right up to it, and you can put the barrel about 20 yards away from them and fire. It's kinda sad how they do the Bison hunts out here, concidering how majestic an animal they are.
Realistically, I wouldn't recommend shooting any game animal out past 200 yards anyways. If you hit it, and it isn't a fatal shot, you have alot of ground to cover while it is going to be moving and you can lose sight of it and end up with a gut shot animal getting away and dying in the brush, and that is a tragedy.
I've hunted with the venerable .30-.30, .25-06, and the .308, all of which have taken elk with no problem. I've only ever had to shoot an animal a second time three times, so again, shot placement is paramount!
Bigger, faster, more wham-bam bullets just ruin more meat.
(welcome back ninjaboy hehe)