If your question is how did Leer go pro "the way he did," we have to know how he went pro. I don't know the answer, but my assumption is that he got a deal that offered gear, the right to train with OTC, probably some travel to meets, and maybe some performance bonuses. Maybe it was more than that, I don't know. But just because someone is running "full time" as a pro doesn't mean they're getting truckloads of cash -- or any cash for that matter.
Also, as others have pointed out, it's not just running 3:40. It's running 3:40.06 in the heats at USATF to qualify for the final, a performance that (correctly, as it turns out) suggests a runner who can rise to the occasion and perform well in championship settings. His 3:40.06 ranked him 18th in the U.S. that year; more than half of the runners ranked ahead of him ran their best times in European races.
As for the comparison with Jake Campbell and Mitchel Black, you're probably right that neither is likely to have as successful a career as Leer did. But that's true for the vast majority of runners, even those coming out of D1 with 3:37 or 3:38 to their name. Leer may have been lucky to be running (early in his career) against relatively weak domestic competition -- but still, his string of top-5 finishes at USATF is pretty rare, even for people who started out with better credentials than him.
In terms of the opportunity to "go pro," any runner who comes out of college even today with a 3:40-flat and a finals appearance at USATF will get the opportunity to continue running if they're eager and resourceful. And if it looks like they're progressing along the paths that Leer and Symmonds followed, the opportunities available to them will continue to get better.
But I guess what you're really asking is "How did they know Leer and Symmonds would turn out to be so good?" And I think the answer is they didn't. Tons of runners get tiny contracts each year, and most of them wash out of the sport pretty quickly. The ones that succeed look like smart picks, but that's often just hindsight bias (though Symmonds did seem to have some of that "it" factor right from when he graduated, as I recall).