Of course, not denying there are many fantastic athletes. If rugby was popular I am sure they would produce many great athletes in that sport too. If they had the necessary high standards of skills coaching, tactical knowledge etc. that comes with time, the US would of course become one of the top rugby teams (on par with the other tier 1 nations). I think that should go without saying to anyone who watches US sports.
But the notion that 'A team of SEC college running backs would kill professional Rugby teams lol' is just flat out stupid, not to mention disrespectful. It wouldn't even be true for NFL running backs (some ESPN guy said this like a month ago). Players from each sport are extremely fine tuned to the one they play...those calls for individual 'athleticism' that you mention mean that they wouldn't have the same kind of stamina needed to play rugby. And after all, fitness is surely an athletic attribute too, not just strength and explosiveness. Rugby players have that too, but not to the same kind of max effort, rest, max effort rest etc. level. It is just different. Rugby players, as you know, need to grind it out over 35-65 minutes of the ball being in play over the course of 80 minutes. NFL plays last a few seconds at a time and amount to c. 11 minutes over the course of about 3 hours or so.
Not to mention all the other factors that make this untrue, NFL or CFB running backs wouldn't have the adequate skills in other areas- tackling, passing, kicking and, in union, scrummaging, lineouts, rucks, to compete against top rugby teams. They are hyper-specialised, making them supremely good at one thing, but lacking in other areas. Rugby positions are specialised to a degree, but much more jack of all trades.
Although it is obviously important, like any ball sport at the highest levels rugby isn't an athletics competition. Otherwise the american guy Carlin Isles who, by the 100m time was one of fastest men to step on to a pitch, would have done much better. Instead he couldn't even get a bench spot at the Glasgow Warriors. There was an English sprinter who attempted to play, he had a trial, but similarly never got given a contract. Other factors including skill, positioning, tactical understanding and so on are equally important; same with the NFL, which is why Jarryd Hayne didn't crack it despite clearly having the physical tools...just too difficult to pick up the game at that standard without either growing up playing it, or taking a much longer time to learn and get those other things ingrained.
I suspect we might be taking past each other a bit here though.