I play golf (last handicap was a 6) and run marathons, so I am not very good at either but have an idea what is involved. In my mind, this thread got lost in an apples and oranges debate.
I honestly did not realize that there were different kinds of triathalons but I think what we witnessed in the British Open are two kinds of sporting talent not witnessed in a triathalon. First, assessment of trajectory within the context of distance, wind, and topography. Golf at that level is not recreational golf and it requires incredible abilities to plan a correct shot. It is very difficult, under the pressure of competition, to focus the mind on all of the environmental cues and assess these correctly. Second, control of fine motor movement. Golf is a bit like ballet in that it requires precise control of movement and repetition. The slightest error in repetition has dramatic consequences for where the ball will land and its final position. Remember, these guys are playing your slice on every shot. They are crafting individual shots, shaping them left to right, right to left, with or without backspin, etc.
Ernie Els was on television here recently outlining the demanding exercise program he had been through to get back into shape to play golf after his surgery and it was not light work. It wasn't triathlon training either.
In my mind, golf requires very different skills to what it takes to complete a triathalon or marathon. Personally, I can't handle the swimming in a triathalon and stick to the road instead. These races require a very different kind of mental control and toughness.
There is a reason why the marathon is the closing event of the Olympics. And you will never see anyone, even in moderate condition, die from the exertion of playing a game of golf. Just completing one of these or a demanding Triathalon, especially at or near 50 years of age, is one hell of an accomplishment.
It is noteworthy that less than one-tenth of one percent of Americans at 50 can complete a marathon or triathalon, while golf is the sport of choice for the over 60s. Quite frankly, as someone who takes part in both sports, I think that about says it all.