Was Babe Ruth an athlete?
In his day I'm sure Ruth was considered an athlete. However, Jim Thorpe could have run circles around around the Great Bambino. And any current NFL running-back would totally school Jim Thorpe. For the most part, you can NOT make trans-generational comparisons between athletes, especially as the athlete's respective eras grow further apart.
Today we have athletes with entire teams of training specialists and nutritionists. This is the age of hyperbaric chambers and lab-developed supplements. In most sports, the competitors are more athletic than they have ever been because they
need to be. Understand this. The current level of competition means that the aspiring football or basketball player can less and less afford to adopt a carefree attitude towards his own talent. It's great to marvel at Babe Ruth stepping up to the plate hungover and covered in mustard stains, but that act will not play in today's environment, especially in sports more athletic than baseball (of which there are many).
Now turn to golfers. As a group, they have progressed at a much slower rate in terms of athleticism. Why? Because athleticism is not as central to golf as it is to other sports. I agree that "athleticism" is an ambiguous term, but you must agree that golf does not require the level of overall physical ability that a sport like basketball or water polo does.
With that said, golfers will increasingly need to to focus on training that will improve their physical ability as it relates to golf. In other words, they will need to specialize. Obviously the emerging science-backed interest in biomechanics will facilitate this. Will this specialized training yield more athletic golfers? Yes, but they will still only be athletic on the relative scale of golfers.
Ultimately, athleticism (whatever you think it means) is only a piece of the golfing pie. I believe a conversation on the mental/psychological aspect would be much more enlightening than arguing over whether Phil Mickelson played backup QB back in high school. Look at Phil. Do you see an athlete or do you see a golfer?