If we're simply talking about potential, I think it is much much higher than 10% of the adult male population. Maybe not in some Asian countries or someplace like Somalia, but developed western countries I think it is well over 50%.
As stated earlier, very few are interested in realizing that potential, whether it be through taking steroids, gaining body weight, doing the work or whatever combination of factors is necessary for the individual to achieve this.
For the record, I think most men can achieve this without taking steroids. I've seen plenty of unremarkable people pulling in the 500's and 600's. The common denominator is always ball-busting work and refusal to give up.
Take our forum member @ActivitiesGuy. He's sub-200 lbs and doesn't even really train for strength. He runs a lot and does kettlebell work and yoga. He pulled 500 within a few months of treating the deadlift as a little side-project of his. Could he pull 600 if he committed to it? I absolutely think he could. Maybe it would take him two years and maybe he'd need to put 20 pounds on, but I totally believe he could do it if he so wished.
I have no idea how to judge someone's genetics, but he seems like a pretty normal guy to me.
In my limited observations, I've seen far more people who train hard and consistently, eat for strength and really apply themselves achieve strength feats like this than I've seen people who do everything right and still end up weak. It is just so rare for someone to even attempt to get to this point, even among gym rats, that it is really hard to say what someone's potential really is. You just don't know until you apply yourself and see where you end up.