Perhaps the flaw is identifying the "deliberate practice" as the only dependent variable. For example, I play guitar. I'm probably better than other people who have put in the same amount of practice time than I have, since I've put in 10 years on clarinet. Those ten years of music experience, give me an ability to interpret rythym, understand some music theory, read notes quickly, hear pitch, etc. than someone who is "just picking it up".
Similarly, I know some quite good chess players who play Go as well -- one of whom is the champion of a small Nordic country. A player of Shogi recently switched to chess -- and he's quite good. Or perhaps, a good COBOL programmer could be a good Swift programmer with less effort?
There's very little in the article discussing the role of "related skills" -- and perhaps this can help explain the gap.