I know that kid is busting his butt, but man I hope it is all genetics. My little girl is only three and you can see her abs clearly defined on some days, depending on how insane she is during her play time (which is all day). I actually get a little worried when I can see them but I have to remember that she is RIDICULOUSLY active. The kid is just a non-stop pile of energy. We make sure to feed her as much as she can handle too. Good, real food first. Then she can have any "junk" food she wants.
So yeah, hopefully these parents are trying to make sure these kids are eating too. A lot of things are being developed. not just physically. My daughter's mental development is a large concern of mine whenever I try to make sure she is eating enough, hence why I am like "wtf" when I can see her abs.