Stack Overflow supports an alternative syntax for
this use case:
Our primary landing page is <http://www.example.com>
Again, this is not knowledge that the user can take
with them to other Markdown based sites.
The material you quote is confusing, but what Stack Overflow and GitHub do that Markdown does not define is allowing auto-linking of URLs without the angle brackets. That is, http://ajh.us/ on SO or GH is the same as <http://ajh.us/>; on any Markdown-based system.