If you inspect [0], you'll find that his MD was, in fact, in medicine. What came before that, though, was a PhD in applied math from Caltech, and that immediately after receiving his MD he became chair of Clarkson University's Math and Computer Science department.
Mentioning his MD is a distraction and, as the previous poster commented, suggests that he's something of an amateur. This is very far from the case.
So this goes off on a tangent but I feel it relates to noncentrality [0]. Fokas has a PhD in maths. Being an MD or having gotten an MD 40 years ago is clearly entirely non-central to his career. Calling him Mathematician-MD seems like it is meant to make him seem a lesser mathematician, e.g. by insinuating that this is just something he does part time, and that he can hence be taken less seriously.
I don't know what the poster meant by suggesting 'Mathematician-MD', but it reads weirdly to me for that reason. It's highlighting an attribute of a person that is entirely unrelated to his career or this article. Why if not to denigrate him? The title should be changed to neutrally reflect his position.
An MD is a medical degree. It's possible to have an MD but not practice medicine professionally in the same way someone can earn a law degree (a JD) but choose to not be a practicing lawyer. "Professor" is a academic title for people at a university or advanced teaching institution. Most professors do have terminal degrees in their respective fields like PhDs, MDs, or JDs, so you could also call a Professor with a PhD "Dr. X" instead of "Prof. X". But, "Professor" is generally considered a more prestigious title since its much rarer and harder to get a professorship than get an advanced degree.
Mentioning his MD is a distraction and, as the previous poster commented, suggests that he's something of an amateur. This is very far from the case.
[0] http://www.damtp.cam.ac.uk/user/tf227/