The difference is notability, which is the criteria used whether a distinct Wikipedia article is warranted. There are multiple cited sources that use the "Noble disease" term, there are possibly much fewer similar publications with "Fields medalist disease".
That is not to say that the theory holds water. Flat Earth theory is also notable and has a wiki page, even if blatantly wrong. There is no hyperboloid Earth theory wiki page.
> there are possibly much fewer similar publications with "Fields medalist disease"
Among mathematicians, I wouldn't claim that "Fields medalist disease" is an unusual term, though this term is typically meant to mean "some mathematician got a Fields medal, but did not prove any other deep mathematical result afterwards". There also exists the much more derogatory term "dead wood" for mathematicians who once proved highly celebrated results, but after they got tenured, no they didn't manage to prove anything deep anymore."
Concerning the "crackpot undertone" of "Nobel Disease": lots of mathematicians know the names of achieved mathematicians who got into some eccentric ideas ("eccentric" in the sense of "outside the Overton window") once they got older.
Famous examples for the latter point are Serge Lang
> https://en.wikipedia.org/w/index.php?title=Serge_Lang&oldid=...
or Anatoly Fomenko
> https://en.wikipedia.org/w/index.php?title=Anatoly_Fomenko&o...
Ok, notability is a good argument for having such an article (although I'm not too sure about it really being notable), but the current shape is absolutely misleading.