It was rejected because the users who reviewed the suggested edit thought it was changing the meaning of the answer.
Effectively, the suggested edit added that Drush 9 is Composer-centralized, while it's simply suggested not to install Drush 9 globally (which is possible also with Composer).
It also introduce a grammar error, in the following sentence. (Emphasis on the wrongly changed part is mine.)
A composer based Drupal project is installed by composer create-project drupal-composer/drupal-project
includes Drush 9 locally by default and this is the only recommend way to use Drush 9.
The correct sentence is the original one.
A composer based Drupal installed by composer create-project drupal-composer/drupal-project
includes Drush 9 locally by default and this is the only recommend way to use Drush 9, see https://github.com/drupal-composer/drupal-project.
Inlining a link, or editing it to show its title is fine; rewriting a sentence to make it clearer is fine, but adding a sentence the poster didn't put (and containing a not correct assertion) isn't fine.