On an MediaWiki 1.23, 1.24 and 1.25 (have not checked 1.26) special page "Special:HostStats" is added to "wiki" group on special page "Special:SpecialPages" as desired via the following code.
On MediaWiki 1.27 it now shows up in the "other" section which is the default if nothing is specified. According to the docu it should still be added to the "wiki" group since the code of the extension did not change. So there must be something in the water.