Assumes Correlation and Causation, ignores Social and Economic situations of the twins groups, doesn't account for the fact that identical twins tend to share (being geared towards) more experiences due to being perceived almost as a single "being", ignores cultural differences in upbringing of different-gender twins, doesn't account for indirect training (i.e. strategy, memorization, sequential steps etc. are parts…
Finger length is relevant factor in piano play. Most people dont hear soinds as well as pro-musicians, our ears webe born less good. That would be talent. More generally, fine motor skills are simply not equal between children even when the kid tries hard. The importance of right genes is pretty clear when you look at sports. Different sports favor different body types and if you don't have one, you won't suceed.
I know a couple musicians that have it, one of them lost it in an accident, both studied a lot, and most musicians I know, including some of the most successful ones, don't have this.
It's a good point, but empirically not verifiable, musicians train to identify intervals (Even the ones with perfect hearing), and in a similar discussion in class when I studied for a music degree, the only things the class disagreed were the impact of perfect hearing in the likelihood of becoming a musician, and that there may be some innate ability to maintain rhythm. On everything else we were unanimous in believing there is no such thing as talent.
Now you're free to believe differently, still, people that you think have "talent" will mostly be offended and feel that their hard work is not being recognized. Phelps isn't amazing BECAUSE of his body. His body is just a very small part of what makes him awesome, and though maybe a bit less awesome, he could still be awesome with a different body.