My not totally uneducated guess is that the particular answer in question received so diverse votes because people have different notion of what exactly constitutes a German word, more specifically people will answer the following question differently: Is a word that is not used in a considerable manner but formed with certain mechanisms a German word or not? If you answer this with yes, the answer in question is okay; if you answer this question with no, it is clearly wrong.
This is also reflected by the comments arguing against the answer:
Das ist doch Käse. Nach Deiner Logik kann ich "-ist" an alle Wörter anhängen und das neue Wort ist automatisch ein deutsches Wort? Bananist? Tigerist? Schnapsist? Hemdist?
"Es ist allerdings kein besonders häufiges" Leichte Untertreibung, oder? Bisher haben wir genau eine Verwendung in der Bedeutung von "Mitarbeiter einer Praxis" die mal jemand in freier Wildbahn gesehen hat. Und für das Kriterium "ist ein deutsches Wort" gibt es strengere Kriterien als "ein Muttersprachler kann sich den Sinn im Kontext erschliessen", meiner Meinung nach.
So, as a first conclusion, I do not consider the voting behaviour mysterious or going against the fundamental concepts of voting. The downvotes come from people considering the answer to be wrong (and they have arguments to do so) and this is exactly what downvotes are for.
Now, if the upvotes and downvotes are based on a different interpretation of the question, is the question to blame? In my opinion, at most partially. We can hardly expect all askers to have the linguistic prowess to know the difficulties of defining German word or to give a definition for the purpose of their question.
Therefore a good answer to this question should address exactly the linguistic problems that make it difficult to give a clear yes-or-no answer or at least state on which definition of German word the answer is based. The answer in question fails to this and thus deserves its balanced score – in my opinion.