The "How to ask" pages are shared between all the SE sites, just branded differently with the site's colors, logos, etc. Private Beta refers to one of the stages a site suggested by the Area 51 process before it graduates to a full-fledged site.
Super User is not in private beta, of course. Perhaps it's worth raising an improvement request on MSE to remove this page for graduated sites like SU or SO.
EDIT:
After searching a bit, seems that this was already reported on MSE back in January.