A problem I see with this argument is that it only takes grappling into account (and even there it is debatable whether short legs are as big a determinant in grappling as the article implies). Apes don't fight exclusively by grappling. It is natural for them to swing their arms and sticks, which make reach and upper-body strength other factors in fighting. Serious gorilla fights are settled by biting. (And Tarzan used a knife as an equalizer of course, LOL.) But most gorilla fights don't get that serious: intimidation ends most fights before they begin, which is the purpose of the chest-thumping, etc. (" Because of their sharp canines and great strength, gorilla males are very dangerous opponents when they are in a fighting mood. Therefore, most differences of opinion are not resolved by fighting, but by displaying and other threat behaviour patterns, without anyone getting injured. Gorillas are famous for their display behaviour culminating in the chest beating and loud hooting. The chestbeat sounds especially impressive in silverback males. Severe aggression is very rare in stable gorilla groups, but when two groups meet, the leading silverbacks can sometimes engage in a fight to the death, mainly using their sharp canines to cause deep, gaping injuries.": http://www.berggorilla.org/english/faq/dvers/fragen/kampf.html) So IMO the real survival advantage here would be the ability to out-intimidate the opponent, not sheer grappling ability.
In addition, short legs have previously been attributed to arboreal ancestors. I regard this as speculation at best. :')