Apparently, the specific essay that Campbell had in mind was "On the Foundations of Morality" (Über die Grundlage der Moral). It's not obscure, but I didn't find it in English, for free, on the web.
It seems that Campbell's first recorded use of the anecdote was in April 1974 at an academic conference in Texas. That's when he mentioned which essay. The attempted suicide would have occurred during the summer of 1971, or thereabouts, "three summers" before the conference. Unfortunately, Hawaiian newspapers from that era are not well represented on the web, as we all know from searching for the Obama birth announcement .
A firefighter who somehow knew that he will die on his next shift may well think, "OK, if I stay home, then who dies in my place?"
Of course, in some religions, it's not only just swell but mandatory to have somebody else die in your place. But maybe that's harder to do when you know the person, and you know that if the situation was reversed, that he wouldn't call in sick.
It's heroic to stand your duty, like the second police officer, but it's a different species of heroism than what's in Campbell's story. The first officer had no duty to the jumper, but he stood anyway.
What happened was not the contraction of awareness that comes from instinct, IMO. What Campbell says the officer said was "... If I had let that young man go, I couldn't have lived another day of my life." That's not contraction of awareness, that's expansion of consciousness. The officer says that he saw the situation in a larger context, the context of his whole life, not the range of the moment decision making that drives thoughtless or instinctual reaction.
And training? Training says not to let a jumper take anyone with him, especially not you. Meh, so much for training.