From the question What are "Community Wiki" posts? we find this exerpt:
How does a post become a Community Wiki post?
There are several ways a question or answer can enter community wiki
mode, and most of these ways will occur automatically based on the
rules of the system.
Posts enter community wiki mode when one of the following happens:
The body of the post has been edited by at least five (5) different users.
The post has been edited ten (10) times by the original owner.
The answer's author checks the community wiki checkbox when composing the answer. Note that this checkbox isn't available to new
The answer is edited by its original author, who when doing so opts to check the community wiki.
An answer posted to a Community Wiki question will also be Community Wiki. Note that when a question is made CW after answers have already
been posted, the existing answers are not normally converted to CW
The question generates more than 30 answers (15 on Super User, 60 on Code Golf). In this case, the question and all answers
will enter community mode, as will any future answers.
A moderator has reason to believe that the question serves better in community wiki mode - if you believe your question should
to a Community Wiki, you may flag it for moderator
* note that the rules outlined above are based on observations; the
actual behavior of SO is subject to change without notice, and may
well have done so since this was last updated...
A brief viewing of the history of your linked question shows that it was edited (as of my last viewing) 17 times by the original owner. So I'm guessing that's why.