I don't think that was the reason for the creed. It was developed to address the decreased powder charge in the 260 with long bullets at S/A mag lengths.
That's often repeated, but doesn't make any sense if you think about it.
If the same bullet in a 260 and a 6.5C are both limited by the same mag length, then they're in the same position relative to the case head, right? The bullet base in the 260 might be back into the powder a bit, but in comparison the 6.5C has the bullet
and the shoulder back into the powder space. I'm not sure why people think setting the shoulder back somehow makes up for powder capacity, when it does exactly the opposite.
Setting the shoulder back for the 6.5C moved the neck back, avoiding seating bullets in the "donut" at the neck/shoulder junction, also (ideally) bringing the throat back as well, for long match bullets to reach the lands at mag length.
