The same applies to the top-flight programmers I know, though fewer seem to take up visual art forms vs. music or writing. Just a theory, but perhaps the talents that lead to programming excellence overlap more with non-visual expression.
But that is the key: the best in any field very likely have more than one talent, so only natural to do a number of seemingly unrelated things well.
While I'm hardly in the league of the individuals mentioned in the article, I have contributed worthy if modest accomplishments. Like the subset of physician colleagues juggling "parallel" artistic careers, I'm also a professional artist (printmaking, photography), and not coincidentally pretty handy at manipulating computers as well.
It seems entirely natural for energetic, curious, motivated and gifted people to enjoy using their abilities in diverse ways. It follows that the most capable people will be show it in several domains.
A final thought is the experience of intense interest in seemingly diverse fields is probably not as discontinuous as others may think. People often say to me art must be a nice diversion from practicing medicine. Actually for me going from one to the other is seamless, there are common elements among my whole range of activities, the essential problem-solving core is not different. Yes the goals and technologies are unmistakeably distinct, yet putting some part of myself into every task is a constant.