And besides, Abdi and Meb (especially Meb) ARE mixing it up on a world level, or at least they have in the past. They may not be medalists in major championships, but with accomplishments like Meb's top 15 World XC, 27:13 or whatever 10,000, and so on, and Abdi usually right on his tail, you would have to concede that they are world class athletes.
But overall, I think there is some validity to your point. African-Americans as a whole may be better suited to sprinting, but I am sure that there are plenty among them who have the makings of great middle and long-distance runners. But there are numerous obstacles they have to get over first. I would guess the primary one would be a lack of social acceptance for distance runners. OK, I'm not black so I'm just theorizing here, but I get the feeling that being an up-and-coming distance runner isn't the best way to win acceptance in your community when you're an African-American. Being a good basketball player or football player seems to be the way to go. Even sprinting seems to be just a sideline sport for African-Americans--something football and basketball stars can do to stay in shape during the offseason.
Also, I'm pretty sure that what you said is true: African-American kids are assumed to be sprinters, and white kids are assumed to be distance guys. Many coaches and many aspiring athletes buy into this way of thinking by the percentages. I would imagine that there could be more successful white sprinters out there, but whites too fall into social stereotyping. I don't know if there would be many whites who could be world-class 100m runners, but they should probably at least be able to mix it up more at 400m, as shown by occasional examples such as Thomas Schonlebe of Germany, and the two Minnesota guys.
So basically, I think African-Americans could produce some top distance runners, but there is little interest in it on anyone's part. I don't think it's an example of any kind of hard-core racism; it's more that for a variety of reasons, African-Americans aren't particularly trying to become distance running stars. By the same token, there are lots of other national or social groups who haven't achieved their potential in a variety of sports: Mexican tennis players, Kenyan polo players, Chinese cricket players, etc.