As the post on MSO about this indicates, /helpcenter/helpcenter is not meant to be read all at once. The individual parts of it are meant to be used as the resource when we refer to it in comments and the such.
/about/about on the other hand contains all the information important to a new user - That's why it's referred to as "the tour" now - an introduction to the site, how it works and what to look for.
The old "What's on-topic" part, for example, is also on /about, in the "Get answers to practical, detailed questions" section.