Keep reading! That is just the Purpose section, which summarizes the license. The specific sections affecting copyleft are Copyleft, Prototypes, Applications, and Contributing. I think they are slightly easier to read and understanding on the master branch of the development repository: https://github.com/kemitchell/api-copyleft-license/blob/mast...

> The purpose of this rule is to encourage cooperative development of this software, minimizing duplication of effort across competing substitutes.

Ah, I understand it now! Thank you.