Nature vs. Nurture. Thats the only component that deems the study effectiveness. The percentage is simply higher because they are in the same environment. Which would increase the chances of both becoming homos3xual, thus making it a learned behavior. Now if they did a study on identical twins in which they split them up during birth, resulting in both becoming homos3xual without ever knowing each other, then an argument could be made...
I'm not really arguing that Homos3xuality is solely attributed to one or two genetic alterations. There are likely a number of other genetic factors that manipulate s3xuality and can predispose towards heteros3xuality or homos3xuality or bis3xuality, in play w/environmental factors. When looking at genetic correlations, experts usually rely on things like relative risk to determine the potential of a disease/characteristic to occur within a family versus what is seen in the general population. Here, with a near 55% concordance in identical twins, is a pretty high degree, especially in comparison to the 11% seen in the adopted brothers. I'm not arguing that something as complex as s3xuality is attributed solely to a few easily tractable genetic "mutations". However, what I stated above and is that the relevant data support that genetics play an important role in this situation, which is definitely in contrast to the "it's solely a learned behavior" crowd